Membrane for continuous analyte sensors

ABSTRACT

Devices are presented for measurement of an analyte concentration. The devices comprise: a sensor configured to generate a signal indicative of a concentration of an analyte; and a sensing membrane located over the sensor. The sensing membrane comprises an enzyme domain comprising an enzyme, a base polymer, and a hydrophilic polymer which makes up from about 5 wt. % to about 30 wt. % of the enzyme domain.

INCORPORATION BY REFERENCE TO RELATED APPLICATION

Any and all priority claims identified in the Application Data Sheet, orany correction thereto, are hereby incorporated by reference under 37CFR 1.57. This application is a continuation of U.S. application Ser.No. 13/836,530, filed Mar. 15, 2013. The aforementioned application isincorporated by reference herein in its entirety, and is herebyexpressly made a part of this specification.

FIELD OF THE INVENTION

Devices are presented for measurement of an analyte concentration. Thedevices comprise: a sensor configured to generate a signal indicative ofa concentration of an analyte; and a sensing membrane located over thesensor. The sensing membrane comprises an enzyme domain comprising anenzyme, a base polymer, and a hydrophilic polymer which makes up fromabout 5 wt. % to about 30 wt. % of the enzyme domain.

BACKGROUND OF THE INVENTION

Electrochemical sensors are useful in chemistry and medicine todetermine the presence or concentration of a biological analyte. Suchsensors are useful, for example, to monitor glucose in diabetic patientsand lactate during critical care events. A variety of intravascular,transcutaneous and implantable sensors have been developed forcontinuously detecting and quantifying blood analytes, such as bloodglucose levels.

However, performance of enzymatic glucose sensors is affected by theamount of oxygen present at the electrode surface. For example, inenzymatic glucose sensors which rely on oxygen as an electron mediator,sensor signal decreases under low oxygen conditions (such as at about0.25 mg/L or lower) for the same glucose concentration. Unfortunately,these performance issues are amplified in sensors with increased glucosesensitivity. As enzymatic glucose sensors capable of increased glucosesensitivity are developed, there is a desire for improved sensorperformance under low oxygen conditions.

Additionally, the enzymes used in enzymatic glucose sensors aresensitive to temperature and pH degradation. Thus, the processes formanufacturing enzymatic glucose sensors are required to be conductedunder pH and temperature conditions which preserve enzymatic activity.For example, polymer membrane curing that occurs after application of anenzyme layer is limited to temperatures at which the enzyme does notdegrade, which are typically well below preferred curing temperaturesfor the polymer. Curing polymer membranes at these restrictedtemperatures extends curing time, increasing cost and limitingthroughput. Accordingly, there is a desire for enzyme stabilization inenzymatic glucose sensors so as to provide enhancement in the tolerablepH range and increased thermal stability in order to decreasemanufacturing time and cost.

SUMMARY OF THE INVENTION

In a first aspect, a device is provided for measurement of an analyteconcentration, the device comprising: a sensor configured to generate asignal indicative of a concentration of an analyte; and a sensingmembrane located over the sensor, the sensing membrane comprising anenzyme domain comprising an enzyme, a base polymer, and a hydrophilicpolymer. In devices of the first aspect, the hydrophilic polymercomprises from about 5 wt. % to about 30 wt. % of the enzyme domain,such as about 10 wt. % to about 25 wt. % of the enzyme domain, such asfrom about 15 wt. % to about 20 wt. % of the enzyme domain.

In some embodiments, the hydrophilic polymer is selected from the groupconsisting of poly-N-vinylpyrrolidone (PVP), poly(ethylene glycol)(PEG), polyacrylamide, acetates, polyethylene oxide (PEO),polyethylacrylate (PEA), poly-N-vinyl-3-ethyl-2-pyrrolidone,poly-N-vinyl-4,5-dimethyl-2-pyrrolidone, poly-N,N-dimethylacrylamide,polyvinyl alcohol, polyvinyl acetate, polymers with pendent ionizablegroups and copolymers or blends thereof. In some embodiments, thehydrophilic polymer comprises poly-N-vinylpyrrolidone (PVP).

In some embodiments, the enzyme is selected from the group consisting ofglucose oxidase, glucose dehydrogenase, galactose oxidase, cholesteroloxidase, amino acid oxidase, alcohol oxidase, lactate oxidase, anduricase. In some embodiments, the enzyme is glucose oxidase.

In some embodiments, the base polymer comprises at least one polymerselected from the group consisting of epoxies, polyolefins,polysiloxanes, polyethers, acrylics, polyesters, carbonates, andpolyurethanes. In some related embodiments, the polyurethanes comprise apolyurethane copolymer. In some embodiments, the base polymer comprisespolyurethane.

In some embodiments, the enzyme domain further comprises a cross-linkingagent in an amount sufficient to induce cross-linking between polymermolecules. In some related embodiments, the cross-linking agentcomprises a cross-linking agent selected from the group consisting ofisocyanate, carbodiimide, gluteraldehyde or other aldehydes, epoxy,acrylates, free-radical based agents, ethylene glycol diglycidyl ether(EGDE), poly(ethylene glycol) diglycidyl ether (PEGDE), and dicumylperoxide (DCP). In other related embodiments, the cross-linking agentcomprises from about 0.1 wt. % to about 15 wt. % of the total dry weightof the enzyme, cross-linking agent, and polymers.

In some embodiments, the thickness of the enzyme domain is from about0.05 micron to about 100 microns.

In some embodiments, the sensor comprises an electrode.

In some embodiments, the sensing membrane further comprises a resistancedomain configured to control a flux of said analyte therethrough.

In some embodiments, the sensing membrane further comprises aninterference domain located more proximal to the sensor than the enzymedomain, wherein the interference domain comprises at least about 25%silicone by weight.

In some embodiments, the device is configured for continuous measurementof an analyte concentration. In some embodiments, the device isconfigured for in vivo intermittent or continuous measurement of ananalyte concentration.

In some embodiments, the device is configured for glucose measurement,including continuous glucose measurement. In some embodiments, thedevice is configured for in vivo intermittent or continuous glucosemeasurement.

In a second aspect, a device is provided for measurement of an analyteconcentration, the device comprising: a sensor configured to generate asignal indicative of a concentration of an analyte; and a sensingmembrane located over the sensor, the sensing membrane comprising anenzyme domain comprising an enzyme, a base polymer, and a dipolar enzymestabilizing agent.

In some embodiments, the enzyme stabilizing agent reduces thermal enzymedegradation, pH-related enzyme degradation, or both.

In some embodiments, the dipolar enzyme stabilizing agent comprises azwitterionic enzyme stabilizing agent. In some related embodiments, thezwitterionic enzyme stabilizing agent comprises a zwitterionic compound,precursor, or derivative thereof. In further related embodiments, thezwitterionic enzyme stabilizing agent comprises a betaine compound orderivative thereof, such as a carboxyl, sulfo, or phosphor betainecompound, precursor, or derivative thereof. In some embodiments, thezwitterionic enzyme stabilizing agent comprises one or more selectedfrom the group consisting of cocamidopropyl betaine, oleamidopropylbetaine, lauryl sulfobetaine, myristyl sulfobetaine, betaine(trimethylglycine), octyl betaine, phosphatidylcholine, glycine betaine,poly(carboxybetaine) (pCB), poly(sulfobetaine) (pSB), and precursors orderivatives thereof. In some embodiments, the one or more thezwitterionic compounds or derivatives thereof comprise one or moreselected from the group consisting of poly(carboxybetaine) (pCB),poly(sulfobetaine) (pSB), and precursors or derivatives thereof.

In some embodiments, the zwitterionic enzyme stabilizing agent comprisesa betaine or derivative thereof, such as glycine betaine. In someembodiments, the betaine or derivative thereof is a hydrolyzablecationic betaine ester. In some related embodiments, the hydrolyzablecationic betaine ester is a cationic poly(carboxybetaine) (pCB) ester.

It will be appreciated that the above listing of zwitterionic compoundsis by no means complete, and is not intended to be limiting. It isintended that other suitable zwitterionic compounds may be recognized bythose of skill in the art. By way of further example, additionalzwitterionic compounds, precursors, or derivatives thereof may includeone or more selected from the group consisting of phosphorylcholine,phosphoryl ethanolamine, phosphatidyl ethanolamine, phosphoethanolamine,phosphatidyl serine, and precursors or derivatives thereof.

In some embodiments, the dipolar enzyme stabilizing agent comprises anon-zwitterionic enzyme stabilizing agent. In some related embodiments,the dipolar enzyme stabilizing agent comprises an amine oxide.

In some embodiments, the dipolar enzyme stabilizing agent is coated onthe surface of said enzyme domain. In some embodiments, the dipolarenzyme stabilizing agent is dispersed throughout the enzyme domain. Insome embodiments, the amount of dipolar enzyme stabilizing agent is lessthan or equal to about the dry weight of enzyme used in the enzymedomain, such as less than or equal 90%, 80%, 70%, 60%, 50%, 40%, 30%,25%, 20%, 15%, 10%, 5%, 2%, or 1%, of the dry weight of enzyme used inthe enzyme domain.

In some embodiments, the enzyme is selected from the group consisting ofglucose oxidase, glucose dehydrogenase, galactose oxidase, cholesteroloxidase, amino acid oxidase, alcohol oxidase, lactate oxidase, anduricase. In some embodiments, the enzyme is glucose oxidase.

In some embodiments, the base polymer comprises at least one polymerselected from the group consisting of epoxies, polyolefins,polysiloxanes, polyethers, acrylics, polyesters, carbonates, andpolyurethanes. In some related embodiments, the polyurethanes comprise apolyurethane copolymer. In some embodiments, the base polymer comprisesa polyurethane.

In some embodiments, the enzyme domain further comprises a cross-linkingagent in an amount sufficient to induce cross-linking between polymermolecules. In some related embodiments, the cross-linking agentcomprises a cross-linking agent selected from the group consisting ofisocyanate, carbodiimide, gluteraldehyde or other aldehydes, epoxy,acrylates, free-radical based agents, ethylene glycol diglycidyl ether(EGDE), poly(ethylene glycol) diglycidyl ether (PEGDE), and dicumylperoxide (DCP). In other related embodiments, the cross-linking agentcomprises from about 0.1 wt. % to about 15 wt. % of the total dry weightof the enzyme, cross-linking agent, and polymers.

In some embodiments, the thickness of the enzyme domain is from about0.05 micron to about 100 microns.

In some embodiments, the sensor comprises an electrode.

In some embodiments, the sensing membrane further comprises a resistancedomain configured to control a flux of said analyte therethrough.

In some embodiments, the sensing membrane further comprises aninterference domain located more proximal to the sensor than the enzymedomain, wherein the interference domain comprises at least about 25%silicone by weight.

In some embodiments, the device is configured for continuous measurementof an analyte concentration. In some embodiments, the device isconfigured for in vivo intermittent or continuous measurement of ananalyte concentration.

In some embodiments, the device is configured for glucose measurement,including continuous glucose measurement. In some embodiments, thedevice is configured for in vivo intermittent or continuous glucosemeasurement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side-view schematic illustrating an in vivo portion of ananalyte sensor, in one embodiment.

FIG. 1B is a perspective-view schematic illustrating an in vivo portionof an analyte sensor, in one embodiment.

FIG. 1C is a side-view schematic illustrating an in vivo portion of ananalyte sensor, in another embodiment.

FIGS. 2A-2C are cross-sectional views through the sensor of FIG. 1 online 2-2, illustrating various embodiments of the membrane system.

FIG. 3 is a graph illustrating the components of a signal measured by aglucose sensor (after sensor break-in was complete), in a non-diabeticvolunteer host.

FIG. 4 shows comparative data demonstrating the relative difference insignal in two specific embodiments of the instant invention as comparedto a prior art device in low oxygen (i.e., at 0.25 mg/L oxygen) andambient oxygen environments at different glucose sensitivities. Detailsare provided in Example 2.

FIG. 5 shows comparative data demonstrating the effect of incrementalincrease of PVP content in the enzyme domain of a glucose sensor of oneembodiment on the signal difference from the sensor in a low oxygenenvironment (i.e., at 0.25 mg/L oxygen) relative to ambient oxygenconditions.

FIG. 6 shows comparative data demonstrating the effect of increasing PVPcontent in the enzyme domain of a glucose sensor of one embodiment onthe signal difference from the sensor in a low oxygen environment (i.e.,at 0.25 mg/L oxygen) relative to ambient oxygen conditions. Details areprovided in Example 3.

DETAILED DESCRIPTION

The following description and examples describe in detail some exemplaryembodiments of devices and methods for providing measurement of ananalyte concentration. It should be appreciated that there are numerousvariations and modifications of the devices and methods described hereinthat are encompassed by the present invention. Accordingly, thedescription of a certain exemplary embodiment should not be deemed tolimit the scope of the present invention.

Definitions

In order to facilitate an understanding of the devices and methodsdescribed herein, a number of terms are defined below.

The term ‘analyte’ as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to a substance or chemical constituent in abiological fluid (for example, blood, interstitial fluid, cerebralspinal fluid, lymph fluid, urine, sweat, saliva, etc.) that can beanalyzed. Analytes can include naturally occurring substances,artificial substances, metabolites, or reaction products. In someembodiments, the analyte for measurement by the sensing regions,devices, and methods is glucose. However, other analytes arecontemplated as well, including, but not limited to:acarboxyprothrombin; acylcarnitine; adenine phosphoribosyl transferase;adenosine deaminase; albumin; alpha-fetoprotein; amino acid profiles(arginine (Krebs cycle), histidine/urocanic acid, homocysteine,phenylalanine/tyrosine, tryptophan); andrenostenedione; antipyrine;arabinitol enantiomers; arginase; benzoylecgonine (cocaine);biotinidase; biopterin; c-reactive protein; carnitine; carnosinase; CD4;ceruloplasmin; chenodeoxycholic acid; chloroquine; cholesterol;cholinesterase; conjugated 1-β hydroxy-cholic acid; cortisol; creatinekinase; creatine kinase MM isoenzyme; cyclosporin A; d-penicillamine;de-ethylchloroquine; dehydroepiandrosterone sulfate; DNA (acetylatorpolymorphism, alcohol dehydrogenase, alpha 1-antitrypsin, cysticfibrosis, Duchenne/Becker muscular dystrophy, glucose-6-phosphatedehydrogenase, hemoglobin A, hemoglobin S, hemoglobin C, hemoglobin D,hemoglobin E, hemoglobin F, D-Punjab, beta-thalassemia, hepatitis Bvirus, HCMV, HIV-1, HTLV-1, Leber hereditary optic neuropathy, MCAD,RNA, PKU, Plasmodium vivax, sexual differentiation, 21-deoxycortisol);desbutylhalofantrine; dihydropteridine reductase; diptheria/tetanusantitoxin; erythrocyte arginase; erythrocyte protoporphyrin; esterase D;fatty acids/acylglycines; free β-human chorionic gonadotropin; freeerythrocyte porphyrin; free thyroxine (FT4); free tri-iodothyronine(FT3); fumarylacetoacetase; galactose/gal-1-phosphate;galactose-1-phosphate uridyltransferase; gentamicin; glucose-6-phosphatedehydrogenase; glutathione; glutathione perioxidase; glycocholic acid;glycosylated hemoglobin; halofantrine; hemoglobin variants;hexosaminidase A; human erythrocyte carbonic anhydrase I;17-alpha-hydroxyprogesterone; hypoxanthine phosphoribosyl transferase;immunoreactive trypsin; lactate; lead; lipoproteins ((a), B/A-1, β);lysozyme; mefloquine; netilmicin; phenobarbitone; phenyloin;phytanic/pristanic acid; progesterone; prolactin; prolidase; purinenucleoside phosphorylase; quinine; reverse tri-iodothyronine (rT3);selenium; serum pancreatic lipase; sissomicin; somatomedin C; specificantibodies (adenovirus, anti-nuclear antibody, anti-zeta antibody,arbovirus, Aujeszky's disease virus, dengue virus, Dracunculusmedinensis, Echinococcus granulosus, Entamoeba histolytica, enterovirus,Giardia duodenalisa, Helicobacter pylori, hepatitis B virus, herpesvirus, HIV-1, IgE (atopic disease), influenza virus, Leishmaniadonovani, leptospira, measles/mumps/rubella, Mycobacterium leprae,Mycoplasma pneumoniae, Myoglobin, Onchocerca volvulus, parainfluenzavirus, Plasmodium falciparum, poliovirus, Pseudomonas aeruginosa,respiratory syncytial virus, rickettsia (scrub typhus), Schistosomamansoni, Toxoplasma gondii, Trepenoma pallidium, Trypanosomacruzi/rangeli, vesicular stomatis virus, Wuchereria bancrofti, yellowfever virus); specific antigens (hepatitis B virus, HIV-1);succinylacetone; sulfadoxine; theophylline; thyrotropin (TSH); thyroxine(T4); thyroxine-binding globulin; trace elements; transferrin;UDP-galactose-4-epimerase; urea; uroporphyrinogen I synthase; vitamin A;white blood cells; and zinc protoporphyrin. Salts, sugar, protein, fat,vitamins, and hormones naturally occurring in blood or interstitialfluids can also constitute analytes in certain embodiments. The analytecan be naturally present in the biological fluid or endogenous, forexample, a metabolic product, a hormone, an antigen, an antibody, andthe like. Alternatively, the analyte can be introduced into the body orexogenous, for example, a contrast agent for imaging, a radioisotope, achemical agent, a fluorocarbon-based synthetic blood, or a drug orpharmaceutical composition, including but not limited to: insulin;ethanol; cannabis (marijuana, tetrahydrocannabinol, hashish); inhalants(nitrous oxide, amyl nitrite, butyl nitrite, chlorohydrocarbons,hydrocarbons); cocaine (crack cocaine); stimulants (amphetamines,methamphetamines, Ritalin, Cylert, Preludin, Didrex, PreState, Voranil,Sandrex, Plegine); depressants (barbituates, methaqualone, tranquilizerssuch as Valium, Librium, Miltown, Serax, Equanil, Tranxene);hallucinogens (phencyclidine, lysergic acid, mescaline, peyote,psilocybin); narcotics (heroin, codeine, morphine, opium, meperidine,Percocet, Percodan, Tussionex, Fentanyl, Darvon, Talwin, Lomotil);designer drugs (analogs of fentanyl, meperidine, amphetamines,methamphetamines, and phencyclidine, for example, Ecstasy); anabolicsteroids; and nicotine. The metabolic products of drugs andpharmaceutical compositions are also contemplated analytes. Analytessuch as neurochemicals and other chemicals generated within the body canalso be analyzed, such as, for example, ascorbic acid, uric acid,dopamine, noradrenaline, 3-methoxytyramine (3MT),3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA),5-hydroxytryptamine (5HT), and 5-hydroxyindoleacetic acid (FHIAA).

The phrase ‘continuous (or continual) analyte sensing’ as used herein isa broad term, and is to be given its ordinary and customary meaning to aperson of ordinary skill in the art (and is not to be limited to aspecial or customized meaning), and refers without limitation to theperiod in which monitoring of analyte concentration is continuously,continually, and or intermittently (but regularly) performed, forexample, about every 5 to 10 minutes.

The terms ‘operable connection’, ‘operably connected,’ and ‘operablylinked’ as used herein are broad terms, and are to be given theirordinary and customary meaning to a person of ordinary skill in the art(and are not to be limited to a special or customized meaning), andrefer without limitation to one or more components linked to anothercomponent(s) in a manner that allows transmission of signals between thecomponents. For example, one or more electrodes can be used to detectthe amount of analyte in a sample and convert that information into asignal; the signal can then be transmitted to a circuit. In this case,the electrode is ‘operably linked’ to the electronic circuitry.

The term ‘host’ as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to animals (e.g. humans) and plants.

The terms ‘electrochemically reactive surface’ and ‘electroactivesurface’ as used herein are broad terms, and are to be given theirordinary and customary meaning to a person of ordinary skill in the art(and are not to be limited to a special or customized meaning), andrefer without limitation to the surface of an electrode where anelectrochemical reaction takes place. As one example, in a workingelectrode, H₂O₂ (hydrogen peroxide) produced by an enzyme-catalyzedreaction of an analyte being detected reacts and thereby creates ameasurable electric current. For example, in the detection of glucose,glucose oxidase produces H₂O₂ as a byproduct. The H₂O₂ reacts with thesurface of the working electrode to produce two protons (2H⁺), twoelectrons (2e⁻), and one molecule of oxygen (O₂), which produces theelectric current being detected. In the case of the counter electrode, areducible species, for example, O₂ is reduced at the electrode surfacein order to balance the current being generated by the workingelectrode.

The terms ‘sensing region,’ ‘sensor,’ and ‘sensing mechanism’ as usedherein are broad terms, and are to be given their ordinary and customarymeaning to a person of ordinary skill in the art (and are not to belimited to a special or customized meaning), and refer withoutlimitation to the region or mechanism of a monitoring device responsiblefor the detection of a particular analyte.

The terms ‘raw data stream’ and ‘data stream’ as used herein are broadterms, and are to be given their ordinary and customary meaning to aperson of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to ananalog or digital signal directly related to the measured glucoseconcentration from the glucose sensor. In one example, the raw datastream is digital data in ‘counts’ converted by an A/D converter from ananalog signal (for example, voltage or amps) representative of a glucoseconcentration. The terms broadly encompass a plurality of time spaceddata points from a substantially continuous glucose sensor, whichcomprises individual measurements taken at time intervals ranging fromfractions of a second up to, for example, 1, 2, or 5 minutes or longer.

The term ‘counts’ as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to a unit of measurement of a digital signal.In one example, a raw data stream measured in counts is directly relatedto a voltage (for example, converted by an A/D converter), which isdirectly related to current from the working electrode. In anotherexample, counter electrode voltage measured in counts is directlyrelated to a voltage.

The term ‘electrical potential’ as used herein is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the electrical potentialdifference between two points in a circuit which is the cause of theflow of a current.

The phrase ‘distal to’ as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the spatial relationshipbetween various elements in comparison to a particular point ofreference. For example, some embodiments of a sensor include a membranesystem having a bioprotective domain and an enzyme domain. If the sensoris deemed to be the point of reference and the bioprotective domain ispositioned farther from the sensor than the enzyme domain, then thebioprotective domain is more distal to the sensor than the enzymedomain.

The phrase ‘proximal to’ as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the spatial relationshipbetween various elements in comparison to a particular point ofreference. For example, some embodiments of a device include a membranesystem having a bioprotective domain and an enzyme domain. If the sensoris deemed to be the point of reference and the enzyme domain ispositioned nearer to the sensor than the bioprotective domain, then theenzyme domain is more proximal to the sensor than the bioprotectivedomain.

The terms ‘interferents’ and ‘interfering species’ as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to effectsor species that interfere with the measurement of an analyte of interestin a sensor to produce a signal that does not accurately represent theanalyte measurement. In an exemplary electrochemical sensor, interferingspecies can include compounds with an oxidation potential that overlapswith that of the analyte to be measured.

The term ‘domain’ as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to regions of a membrane that can be layers,uniform or non-uniform gradients (i.e., anisotropic) or provided asportions of the membrane.

The terms ‘sensing membrane’ and ‘membrane system’ as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refers without limitation to apermeable or semi-permeable membrane that can comprise one or moredomains and constructed of materials of a few microns thickness or more,which are permeable to oxygen and may or may not be permeable to ananalyte of interest. In one example, the sensing membrane or membranesystem may comprise an immobilized glucose oxidase enzyme, which enablesan electrochemical reaction to occur to measure a concentration ofglucose.

The term ‘baseline’ as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to the component of an analyte sensor signalthat is not related to the analyte concentration. In one example of aglucose sensor, the baseline is composed substantially of signalcontribution due to factors other than glucose (for example, interferingspecies, non-reaction-related hydrogen peroxide, or other electroactivespecies with an oxidation potential that overlaps with hydrogenperoxide). In some embodiments wherein a calibration is defined bysolving for the equation y=mx+b, the value of b represents the baselineof the signal.

The term ‘sensitivity’ as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to an amount of electricalcurrent produced by a predetermined amount (unit) of the measuredanalyte. For example, in one embodiment, a sensor has a sensitivity (orslope) of from about 1 to about 100 picoAmps of current for every 1mg/dL of glucose analyte.

The term ‘hydrophilic polymer” as used herein is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizemeaning), and refer without limitation to polymers which will absorb inatmospheric conditions more than about 30% of its weight in water undercommon hydrophilicity test conditions. A common measure ofhydrophilicity of polymers is water absorption by the bulk polymerwithin 24 hours or at equilibrium, as detailed in ASTM D570 (standardmethod to measure water absorption by polymers).

The term ‘dipole’ or ‘dipolar compound’ as used herein is a broad term,and is to be given its ordinary and customary meaning to a person ofordinary skill in the art (and is not to be limited to a special orcustomized meaning), and refer without limitation to compounds in whicha neutral molecule of the compound has a positive and negativeelectrical charge at different locations within the molecule. Thepositive and negative electrical charges within the molecule can be anynon-zero charges up to and including full unit charges.

The terms ‘zwitterion’ and ‘zwitterionic compound’ as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and is not to be limited to aspecial or customized meaning), and refer without limitation tocompounds in which a neutral molecule of the compound has a unitpositive and unit negative electrical charge at different locationswithin the molecule. Such compounds are a type of dipolar compounds, andare also sometimes known as ‘inner salts’.

A ‘zwitterion precursor’ or ‘zwitterionic compound precursor’ is anycompound that is not itself a zwitterion, but may become a zwitterion ina final or transition state through chemical reaction. In someembodiments described herein, devices comprise zwitterion precursorsthat may be converted to zwitterions prior to in vivo implantation ofthe device. Alternatively, in some embodiments described herein, devicescomprise zwitterion precursors that may be converted to zwitterions bysome chemical reaction that occurs after in vivo implantation of thedevice.

A ‘zwitterion derivative’ or ‘zwitterionic compound derivative’ is anycompound that is not itself a zwitterion, but rather is the product of achemical reaction where a zwitterion is converted to a non-zwitterion.Such reactions may be reversible, such that under certain conditionszwitterion derivatives may act as zwitterion precursors. For example,hydrolyzable betaine esters formed from zwitterionic betaines arecationic zwitterion derivatives that under the appropriate conditionsare capable of undergoing hydrolysis to revert to zwitterionic betaines.

The terms ‘non-zwitterionic dipole’ and ‘non-zwitterionic dipolarcompound’ as used herein are broad terms, and are to be given theirordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), and referwithout limitation to compounds in which a neutral molecule of thecompound have a positive and negative electrical charge at differentlocations within the molecule. The positive and negative electricalcharges within the molecule can be any non-zero, but less than fullunit, charges.

The term “polyampholytic polymer” as used herein is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to polymers comprising bothcationic and anionic end groups. Such polymers may be prepared to haveabout equal numbers of positive and negative charges, and thus thesurface of such polymers may be about net neutrally charged.Alternatively, such polymers may be prepared to have an excess of eitherpositive or negative charges, and thus the surface of such polymers maybe net positively or negatively charged, respectively.

As employed herein, the following abbreviations apply: Eq and Eqs(equivalents); mEq (milliequivalents); M (molar); mM (millimolar) μM(micromolar); N (Normal); mol (moles); mmol (millimoles); μmol(micromoles); nmol (nanomoles); g (grams); mg (milligrams);(micrograms); Kg (kilograms); L (liters); mL (milliliters); dL(deciliters); μL (microliters); cm (centimeters); mm (millimeters); μm(micrometers); nm (nanometers); h and hr (hours); min. (minutes); s andsec (seconds); ° C. (degrees Centigrade).

Overview

Membrane systems of the various embodiments are suitable for use withimplantable devices in contact with a biological fluid. For example, themembrane systems can be utilized with implantable devices, such asdevices for monitoring and determining analyte levels in a biologicalfluid, for example, devices for monitoring glucose levels forindividuals having diabetes. In some embodiments, the analyte-measuringdevice is a continuous device. The analyte-measuring device can employany suitable sensing element to provide the raw signal, including butnot limited to those involving enzymatic, chemical, physical,electrochemical, spectrophotometric, polarimetric, calorimetric,radiometric, immunochemical, or like elements.

Although some of the description that follows is directed atglucose-measuring devices, including the described membrane systems andmethods for their use, these membrane systems are not limited to use indevices that measure or monitor glucose. These membrane systems aresuitable for use in any of a variety of devices, including, for example,devices that detect and quantify other analytes present in biologicalfluids (e.g., cholesterol, amino acids, alcohol, galactose, andlactate), cell transplantation devices (see, for example, U.S. Pat. Nos.6,015,572, 5,964,745, and 6,083,523), drug delivery devices (see, forexample, U.S. Pat. Nos. 5,458,631, 5,820,589, and 5,972,369), and thelike.

In one embodiment, the analyte sensor is an implantable glucose sensor,such as described with reference to U.S. Pat. No. 6,001,067 and U.S.Patent Publication No. US-2005-0027463-A1. In another embodiment, theanalyte sensor is a glucose sensor, such as described with reference toU.S. Patent Publication No. US-2006-0020187-A1. In still otherembodiments, the sensor is configured to be implanted in a host vesselor extra-corporeally, such as is described in U.S. Patent PublicationNo. US-2007-0027385-A1, U.S. Patent Publication No. US-2008-0119703-A1,U.S. Patent Publication No. US-2008-0108942-A1, and U.S. PatentPublication No. US-2007-0197890-A1. In some embodiments, the sensor isconfigured as a dual-electrode sensor, such as described in U.S. PatentPublication No. US-2005-0143635-A1, U.S. Patent Publication No.US-2007-0027385-A1, U.S. Patent Publication No. US-2007-0213611-A1, andU.S. Patent Publication No. US-2008-0083617-A1. In one alternativeembodiment, the continuous glucose sensor comprises a sensor such asdescribed in U.S. Pat. No. 6,565,509 to Say et al., for example. Inanother alternative embodiment, the continuous glucose sensor comprisesa subcutaneous sensor such as described with reference to U.S. Pat. No.6,579,690 to Bonnecaze et al. or U.S. Pat. No. 6,484,046 to Say et al.,for example. In another alternative embodiment, the continuous glucosesensor comprises a refillable subcutaneous sensor such as described withreference to U.S. Pat. No. 6,512,939 to Colvin et al., for example. Inyet another alternative embodiment, the continuous glucose sensorcomprises an intravascular sensor such as described with reference toU.S. Pat. No. 6,477,395 to Schulman et al., for example. In anotheralternative embodiment, the continuous glucose sensor comprises anintravascular sensor such as described with reference to U.S. Pat. No.6,424,847 to Mastrototaro et al. In some embodiments, the electrodesystem can be used with any of a variety of known in vivo analytesensors or monitors, such as U.S. Pat. No. 7,157,528 to Ward; U.S. Pat.No. 6,212,416 to Ward et al.; U.S. Pat. No. 6,119,028 to Schulman etal.; U.S. Pat. No. 6,400,974 to Lesho; U.S. Pat. No. 6,595,919 to Berneret al.; U.S. Pat. No. 6,141,573 to Kurnik et al.; U.S. Pat. No.6,122,536 to Sun et al.; European Patent Application EP 1153571 toVarall et al.; U.S. Pat. No. 6,512,939 to Colvin et al.; U.S. Pat. No.5,605,152 to Slate et al.; U.S. Pat. No. 4,431,004 to Bessman et al.;U.S. Pat. No. 4,703,756 to Gough et al.; U.S. Pat. No. 6,514,718 toHeller et al.; U.S. Pat. No. 5,985,129 to Gough et al.; WO PatentApplication Publication No. 04/021877 to Caduff; U.S. Pat. No. 5,494,562to Maley et al.; U.S. Pat. No. 6,120,676 to Heller et al.; and U.S. Pat.No. 6,542,765 to Guy et al. In general, it is understood that thedisclosed embodiments are applicable to a variety of continuous analytemeasuring device configurations. In some embodiments, a long term sensor(e.g., wholly implantable or intravascular) is configured and arrangedto function for a time period of from about 30 days or less to about oneyear or more (e.g., a sensor session). In some embodiments, a short termsensor (e.g., one that is transcutaneous or intravascular) is configuredand arranged to function for a time period of from about a few hours toabout 30 days, including a time period of about 1, 2, 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,27, 28 or 29 days (e.g., a sensor session). As used herein, the term‘sensor session’ is a broad term and refers without limitation to theperiod of time the sensor is applied to (e.g., implanted in) the host oris being used to obtain sensor values. For example, in some embodiments,a sensor session extends from the time of sensor implantation (e.g.,including insertion of the sensor into subcutaneous tissue and placingthe sensor into fluid communication with a host's circulatory system) tothe time when the sensor is removed.

Exemplary Glucose Sensor Configurations

FIGS. 1A-1C illustrate one exemplary embodiment of a continuous analytesensor 100, which includes an elongated conductive body 102. Theelongated conductive body 102 includes a core 110 (see FIG. 1B) and afirst layer 112 at least partially surrounding the core. The first layerincludes a working electrode (e.g., located in window 106) and amembrane 108 located over the working electrode configured and arrangedfor multi-axis bending. In some embodiments, the core and first layercan be of a single material (e.g., platinum). In some embodiments, theelongated conductive body is a composite of at least two materials, suchas a composite of two conductive materials, or a composite of at leastone conductive material and at least one non-conductive material. Insome embodiments, the elongated conductive body comprises a plurality oflayers. In certain embodiments, there are at least two concentric (e.g.,annular) layers, such as a core formed of a first material and a firstlayer formed of a second material. However, additional layers can beincluded in some embodiments. In some embodiments, the layers arecoaxial.

The elongated conductive body may be long and thin, yet flexible andstrong. For example, in some embodiments, the smallest dimension of theelongated conductive body is less than about 0.1 inches, 0.075 inches,0.05 inches, 0.025 inches, 0.01 inches, 0.004 inches, or 0.002 inches.While the elongated conductive body is illustrated in FIGS. 1A-1C ashaving a circular cross-section, in other embodiments the cross-sectionof the elongated conductive body can be ovoid, rectangular, triangular,polyhedral, star-shaped, C-shaped, T-shaped, X-shaped, Y-Shaped,irregular, or the like. In one embodiment, a conductive wire electrodeis employed as a core. To such a clad electrode, two additionalconducting layers may be added (e.g., with intervening insulating layersprovided for electrical isolation). The conductive layers can becomprised of any suitable material. In certain embodiments, it can bedesirable to employ a conductive layer comprising conductive particles(i.e., particles of a conductive material) in a polymer or other binder.

In certain embodiments, the materials used to form the elongatedconductive body (e.g., stainless steel, titanium, tantalum, platinum,platinum-iridium, iridium, certain polymers, and/or the like) can bestrong and hard, and therefore are resistant to breakage. For example,in some embodiments, the ultimate tensile strength of the elongatedconductive body is from about 80 kPsi to about 500 kPsi. In anotherexample, in some embodiments, the Young's modulus of the elongatedconductive body is from about 160 GPa to about 220 GPa. In still anotherexample, in some embodiments, the yield strength of the elongatedconductive body is from about 60 kPsi to about 2200 MPa. Ultimatetensile strength, Young's modulus, and yield strength are discussed ingreater detail elsewhere herein. In some embodiments, the sensor's smalldiameter provides (e.g., imparts, enables) flexibility to thesematerials, and therefore to the sensor as a whole. Thus, the sensor canwithstand repeated forces applied to it by surrounding tissue. Onemeasurement of the sensor's ability to withstand the implantationenvironment is fatigue life, which is described in greater detail in thesection entitled “Multi-Axis Bending.” In some embodiments, the fatiguelife of the sensor is at least 1,000 cycles of flexing of from about 28°to about 110° at a bend radius of about 0.125-inches.

In addition to providing structural support, resiliency and flexibility,in some embodiments, the core 110 (or a component thereof) provideselectrical conduction for an electrical signal from the workingelectrode to sensor electronics (not shown), which are describedelsewhere herein. In some embodiments, the core 110 comprises aconductive material, such as stainless steel, titanium, tantalum, aconductive polymer, and/or the like. However, in other embodiments, thecore is formed from a non-conductive material, such as a non-conductivepolymer. In yet other embodiments, the core comprises a plurality oflayers of materials. For example, in one embodiment the core includes aninner core and an outer core. In a further embodiment, the inner core isformed of a first conductive material and the outer core is formed of asecond conductive material. For example, in some embodiments, the firstconductive material is stainless steel, titanium, tantalum, a conductivepolymer, an alloy, and/or the like, and the second conductive materialis conductive material selected to provide electrical conduction betweenthe core and the first layer, and/or to attach the first layer to thecore (e.g., if the first layer is formed of a material that does notattach well to the core material). In another embodiment, the core isformed of a non-conductive material (e.g., a non-conductive metal and/ora non-conductive polymer) and the first layer is a conductive material,such as stainless steel, titanium, tantalum, a conductive polymer,and/or the like. The core and the first layer can be of a single (orsame) material, e.g., platinum. One skilled in the art appreciates thatadditional configurations are possible.

Referring again to FIGS. 1A-1C, in some embodiments, the first layer 112is formed of a conductive material. The working electrode is an exposedportion of the surface of the first layer. Accordingly, the first layeris formed of a material configured to provide a suitable electroactivesurface for the working electrode, a material such as but not limited toplatinum, platinum-iridium, gold, palladium, iridium, graphite, carbon,a conductive polymer, an alloy and/or the like.

As illustrated in FIGS. 1B-1C, a second layer 104 surrounds a least aportion of the first layer 112, thereby defining the boundaries of theworking electrode. In some embodiments, the second layer 104 serves asan insulator and is formed of an insulating material, such as polyimide,polyurethane, parylene, or any other known insulating materials. Forexample, in one embodiment the second layer is disposed on the firstlayer and configured such that the working electrode is exposed viawindow 106. In another embodiment, an elongated conductive body,including the core, the first layer and the second layer, is provided,and the working electrode is exposed (i.e., formed) by removing aportion of the second layer, thereby forming the window 106 throughwhich the electroactive surface of the working electrode (e.g., theexposed surface of the first layer) is exposed. In some embodiments, theworking electrode is exposed by (e.g., window 106 is formed by) removinga portion of the second and (optionally) third layers. Removal ofcoating materials from one or more layers of elongated conductive body(e.g., to expose the electroactive surface of the working electrode) canbe performed by hand, excimer lasing, chemical etching, laser ablation,grit-blasting, or the like.

In some embodiments, the sensor further comprises a third layer 114comprising a conductive material. In further embodiments, the thirdlayer may comprise a reference electrode, which may be formed of asilver-containing material that is applied onto the second layer (e.g.,an insulator). The silver-containing material may include any of avariety of materials and be in various forms, such as, Ag/AgCl-polymerpastes, paints, polymer-based conducting mixture, and/or inks that arecommercially available, for example. The third layer can be processedusing a pasting/dipping/coating step, for example, using a die-metereddip coating process. In one exemplary embodiment, an Ag/AgCl polymerpaste is applied to an elongated body by dip-coating the body (e.g.,using a meniscus coating technique) and then drawing the body through adie to meter the coating to a precise thickness. In some embodiments,multiple coating steps are used to build up the coating to apredetermined thickness. Such a drawing method can be utilized forforming one or more of the electrodes in the device depicted in FIG. 1B.

In some embodiments, the silver grain in the Ag/AgCl solution or pastecan have an average particle size corresponding to a maximum particledimension that is less than about 100 microns, or less than about 50microns, or less than about 30 microns, or less than about 20 microns,or less than about 10 microns, or less than about 5 microns. The silverchloride grain in the Ag/AgCl solution or paste can have an averageparticle size corresponding to a maximum particle dimension that is lessthan about 100 microns, or less than about 80 microns, or less thanabout 60 microns, or less than about 50 microns, or less than about 20microns, or less than about 10 microns. The silver grain and the silverchloride grain may be incorporated at a ratio of the silver chloridegrain:silver grain of from about 0.01:1 to 2:1 by weight, or from about0.1:1 to 1:1. The silver grains and the silver chloride grains are thenmixed with a carrier (e.g., a polyurethane) to form a solution or paste.In certain embodiments, the Ag/AgCl component form from about 10% toabout 65% by weight of the total Ag/AgCl solution or paste, or fromabout 20% to about 50%, or from about 23% to about 37%. In someembodiments, the Ag/AgCl solution or paste has a viscosity (underambient conditions) that is from about 1 to about 500 centipoise, orfrom about 10 to about 300 centipoise, of from about 50 to about 150centipoise.

In some embodiments, Ag/AgCl particles are mixed into a polymer, such aspolyurethane, polyimide, or the like, to form the silver-containingmaterial for the reference electrode. In some embodiments, the thirdlayer is cured, for example, by using an oven or other curing process.In some embodiments, a covering of fluid-permeable polymer withconductive particles (e.g., carbon particles) therein is applied overthe reference electrode and/or third layer. A layer of insulatingmaterial is located over a portion of the silver-containing material, insome embodiments.

In some embodiments, the elongated conductive body further comprises oneor more intermediate layers located between the core and the firstlayer. For example, in some embodiments, the intermediate layer is aninsulator, a conductor, a polymer, and/or an adhesive.

It is contemplated that the ratio between the thickness of the Ag/AgCllayer and the thickness of an insulator (e.g., polyurethane orpolyimide) layer can be controlled, so as to allow for a certain errormargin (e.g., an error margin resulting from the etching process) thatwould not result in a defective sensor (e.g., due to a defect resultingfrom an etching process that cuts into a depth more than intended,thereby unintentionally exposing an electroactive surface). This ratiomay be different depending on the type of etching process used, whetherit is laser ablation, grit blasting, chemical etching, or some otheretching method. In one embodiment in which laser ablation is performedto remove a Ag/AgCl layer and a polyurethane layer, the ratio of thethickness of the Ag/AgCl layer and the thickness of the polyurethanelayer can be from about 1:5 to about 1:1, or from about 1:3 to about1:2.

In certain embodiment, the core comprises a non-conductive polymer andthe first layer comprises a conductive material. Such a sensorconfiguration can sometimes provide reduced material costs, in that itreplaces a typically expensive material with an inexpensive material.For example, in some embodiments, the core is formed of a non-conductivepolymer, such as, a nylon or polyester filament, string or cord, whichcan be coated and/or plated with a conductive material, such asplatinum, platinum-iridium, gold, palladium, iridium, graphite, carbon,a conductive polymer, and allows or combinations thereof.

As illustrated in FIG. 1C, the sensor also includes a membrane 108covering at least a portion of the working electrode. Membranes arediscussed in detail in greater detail elsewhere herein, for example,with reference to FIGS. 2A-2C.

In embodiments wherein an outer insulator is disposed, a portion of thecoated assembly structure can be stripped or otherwise removed, forexample, by hand, excimer lasing, chemical etching, laser ablation,grit-blasting, or the like, to expose the electroactive surfaces.Alternatively, a portion of the electrode can be masked prior todepositing the insulator in order to maintain an exposed electroactivesurface area.

In some embodiments, a radial window is formed through the insulatingmaterial to expose a circumferential electroactive surface of theworking electrode. Additionally, sections of electroactive surface ofthe reference electrode are exposed. For example, the sections ofelectroactive surface can be masked during deposition of an outerinsulating layer or etched after deposition of an outer insulatinglayer. In some applications, cellular attack or migration of cells tothe sensor can cause reduced sensitivity or function of the device,particularly after the first day of implantation. However, when theexposed electroactive surface is distributed circumferentially about thesensor (e.g. as in a radial window), the available surface area forreaction can be sufficiently distributed so as to minimize the effect oflocal cellular invasion of the sensor on the sensor signal.Alternatively, a tangential exposed electroactive window can be formed,for example, by stripping only one side of the coated assemblystructure. In other alternative embodiments, the window can be providedat the tip of the coated assembly structure such that the electroactivesurfaces are exposed at the tip of the sensor. Other methods andconfigurations for exposing electroactive surfaces can also be employed.

In some alternative embodiments, additional electrodes can be includedwithin the assembly, for example, a three-electrode system (working,reference, and counter electrodes) and an additional working electrode(e.g. an electrode which can be used to generate oxygen, which isconfigured as a baseline subtracting electrode, or which is configuredfor measuring additional analytes). U.S. Pat. No. 7,081,195, U.S. PatentPublication No. US-2005-0143635-A1 and U.S. Patent Publication No.US-2007-0027385-A1, each of which are incorporated herein by reference,describe some systems and methods for implementing and using additionalworking, counter, and reference electrodes. In one implementationwherein the sensor comprises two working electrodes, the two workingelectrodes are juxtapositioned, around which the reference electrode isdisposed (e.g. helically wound). In some embodiments wherein two or moreworking electrodes are provided, the working electrodes can be formed ina double-, triple-, quad-, etc. helix configuration along the length ofthe sensor (for example, surrounding a reference electrode, insulatedrod, or other support structure). The resulting electrode system can beconfigured with an appropriate membrane system, wherein the firstworking electrode is configured to measure a first signal comprisingglucose and baseline signals, and the additional working electrode isconfigured to measure a baseline signal consisting of the baselinesignal only. In these embodiments, the second working electrode may beconfigured to be substantially similar to the first working electrode,but without an enzyme disposed thereon. In this way, the baseline signalcan be determined and subtracted from the first signal to generate adifference signal, i.e., a glucose-only signal that is substantially notsubject to fluctuations in the baseline or interfering species on thesignal, such as described in U.S. Patent Publication No.US-2005-0143635-A1, U.S. Patent Publication No. US-2007-0027385-A1, andU.S. Patent Publication No. US-2007-0213611-A1, and U.S. PatentPublication No. US-2008-0083617-A1, which are incorporated herein byreference in their entirety.

It has been found that in some electrode systems involving two workingelectrodes, i.e., in some dual-electrode systems, the working electrodesmay sometimes be slightly different from each other. For instance, twoworking electrodes, even when manufactured from a single facility mayslightly differ in thickness or permeability because of the electrodes'high sensitivity to environmental conditions (e.g. temperature,humidity) during fabrication. Accordingly, the working electrodes of adual-electrode system may sometimes have varying diffusion, membranethickness, and diffusion characteristics. As a result, theabove-described difference signal (i.e., a glucose-only signal,generated from subtracting the baseline signal from the first signal)may not be completely accurate. To mitigate this, it is contemplatedthat in some dual-electrode systems, both working electrodes may befabricated with one or more membranes that each includes a bioprotectivelayer, which is described in more detail elsewhere herein.

It is contemplated that the sensing region may include any of a varietyof electrode configurations. For example, in some embodiments, inaddition to one or more glucose-measuring working electrodes, thesensing region may also include a reference electrode or otherelectrodes associated with the working electrode. In these particularembodiments, the sensing region may also include a separate reference orcounter electrode associated with one or more optional auxiliary workingelectrodes. In other embodiments, the sensing region may include aglucose-measuring working electrode, an auxiliary working electrode, twocounter electrodes (one for each working electrode), and one sharedreference electrode. In yet other embodiments, the sensing region mayinclude a glucose-measuring working electrode, an auxiliary workingelectrode, two reference electrodes, and one shared counter electrode.

U.S. Patent Publication No. US-2008-0119703-A1 and U.S. PatentPublication No. US-2005-0245799-A1 describe additional configurationsfor using the continuous sensor in different body locations. In someembodiments, the sensor is configured for transcutaneous implantation inthe host. In alternative embodiments, the sensor is configured forinsertion into the circulatory system, such as a peripheral vein orartery. However, in other embodiments, the sensor is configured forinsertion into the central circulatory system, such as but not limitedto the vena cava. In still other embodiments, the sensor can be placedin an extracorporeal circulation system, such as but not limited to anintravascular access device providing extracorporeal access to a bloodvessel, an intravenous fluid infusion system, an extracorporeal bloodchemistry analysis device, a dialysis machine, a heart-lung machine(i.e., a device used to provide blood circulation and oxygenation whilethe heart is stopped during heart surgery), etc. In still otherembodiments, the sensor can be configured to be wholly implantable, asdescribed in U.S. Pat. No. 6,001,067.

FIG. 2A is a cross-sectional view through the sensor of FIG. 1A on line2-2, illustrating one embodiment of the membrane system 208. In thisparticular embodiment, the membrane system includes an interferencedomain 242, an enzyme domain 244, and a diffusion resistance domain 246located around the working electrode 238, all of which are described inmore detail elsewhere herein.

As illustrated in FIG. 2B, in some embodiments, the membrane system mayinclude a bioprotective domain 248, also referred to as acell-impermeable domain or biointerface domain, comprising asurface-modified base polymer as described in more detail elsewhereherein. In some embodiments, a unitary diffusion resistance domain andbioprotective domain may be included in the membrane system (e.g.,wherein the functionality of both domains is incorporated into onedomain, i.e., the bioprotective domain). In some embodiments, the sensoris configured for short-term implantation (e.g., from about 1 to 30days). However, it is understood that the membrane system 208 can bemodified for use in other devices, for example, by including only one ormore of the domains, or additional domains.

As illustrated in FIG. 2C, in some embodiments, the membrane system mayinclude an electrode domain 236. The electrode domain 236 is provided toensure that an electrochemical reaction occurs between the electroactivesurfaces of the working electrode and the reference electrode, and thusthe electrode domain may be situated more proximal to the electroactivesurfaces than the interference and/or enzyme domain. The electrodedomain may include a coating that maintains a layer of water at theelectrochemically reactive surfaces of the sensor. In other words, theelectrode domain may be present to provide an environment between thesurfaces of the working electrode and the reference electrode, whichfacilitates an electrochemical reaction between the electrodes.

A wide variety of configurations and combinations for the various layersin the membrane system are encompassed by the preferred embodiments. Invarious embodiments, any of the domains illustrated in FIGS. 2A-2C maybe omitted, altered, substituted for, and/or incorporated togetherwithout departing from the spirit of the preferred embodiments. It is tobe understood that sensing membranes modified for other sensors, forexample, may include fewer or additional layers. For example, in someembodiments, the membrane system may comprise one electrode layer, oneenzyme layer, and two bioprotective layers, but in other embodiments,the membrane system may comprise one electrode layer, two enzyme layers,and one bioprotective layer. In some embodiments, the bioprotectivelayer may be configured to function as the diffusion resistance domainand control the flux of the analyte (e.g., glucose) to the underlyingmembrane layers.

In some embodiments, one or more domains of the sensing membranes may beformed from materials such as silicone, polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, homopolymers,copolymers, terpolymers of polyurethanes, polypropylene (PP),polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polybutyleneterephthalate (PBT), polymethylmethacrylate (PMMA), polyether etherketone (PEEK), polyurethanes, cellulosic polymers, poly(ethylene oxide),poly(propylene oxide) and copolymers and blends thereof, polysulfonesand block copolymers thereof including, for example, di-block,tri-block, alternating, random and graft copolymers.

In some embodiments, the sensing membrane can be deposited on theelectroactive surfaces of the electrode material using known thin orthick film techniques (for example, spraying, electro-depositing,dipping, or the like). It should be appreciated that the sensingmembrane located over the working electrode does not have to have thesame structure as the sensing membrane located over the referenceelectrode; for example, the enzyme domain deposited over the workingelectrode does not necessarily need to be deposited over the referenceor counter electrodes.

Although the exemplary embodiments illustrated in FIGS. 2A-2C involvecircumferentially extending membrane systems, the membranes describedherein may be applied to any planar or non-planar surface, for example,the substrate-based sensor structure of U.S. Pat. No. 6,565,509 to Sayet al.

Sensor Electronics

In general, analyte sensor systems have electronics associatedtherewith, also referred to as a ‘computer system’ that can includehardware, firmware, or software that enable measurement and processingof data indicative of analyte levels in the host. In one exemplaryembodiment of an electrochemical sensor, the electronics include apotentiostat, a power source for providing power to the sensor, andother components useful for signal processing. In additionalembodiments, some or all of the electronics can be in wired or wirelesscommunication with the sensor or other portions of the electronics. Forexample, a potentiostat disposed on the device can be wired to theremaining electronics (e.g. a processor, a recorder, a transmitter, areceiver, etc.), which reside on the bedside. In another example, someportion of the electronics is wirelessly connected to another portion ofthe electronics (e.g., a receiver), such as by infrared (IR) or RF. Itis contemplated that other embodiments of electronics may be useful forproviding sensor data output, such as those described in U.S. PatentPublication No. US-2005-0192557-A1, U.S. Patent Publication No.US-2005-0245795-A1; U.S. Patent Publication No. US-2005-0245795-A1, andU.S. Patent Publication No. US-2005-0245795-A1, U.S. Patent PublicationNo. US-2008-0119703-A1, and U.S. Patent Publication No.US-2008-0108942-A1, each of which is incorporated herein by reference intheir entirety.

In one preferred embodiment, a potentiostat is operably connected to theelectrode(s) (such as described elsewhere herein), which biases thesensor to enable measurement of a current signal indicative of theanalyte concentration in the host (also referred to as the analogportion). In some embodiments, the potentiostat includes a resistor thattranslates the current into voltage. In some alternative embodiments, acurrent to frequency converter is provided that is configured tocontinuously integrate the measured current, for example, using a chargecounting device. In some embodiments, the electronics include an A/Dconverter that digitizes the analog signal into a digital signal, alsoreferred to as ‘counts’ for processing. Accordingly, the resulting rawdata stream in counts, also referred to as raw sensor data, is directlyrelated to the current measured by the potentiostat.

In general, the electronics include a processor module that includes thecentral control unit that controls the processing of the sensor system.In some embodiments, the processor module includes a microprocessor,however a computer system other than a microprocessor can be used toprocess data as described herein, for example an ASIC can be used forsome or all of the sensor's central processing. The processor typicallyprovides semi-permanent storage of data, for example, storing data suchas sensor identifier (ID) and programming to process data streams (forexample, programming for data smoothing or replacement of signalartifacts such as is described in U.S. Patent Publication No.US-2005-0043598-A1). The processor additionally can be used for thesystem's cache memory, for example for temporarily storing recent sensordata. In some embodiments, the processor module comprises memory storagecomponents such as ROM, RAM, dynamic-RAM, static-RAM, non-static RAM,EEPROM, rewritable ROMs, flash memory, and the like.

In some embodiments, the processor module comprises a digital filter,for example, an infinite impulse response (IIR) or finite impulseresponse (FIR) filter, configured to smooth the raw data stream.Generally, digital filters are programmed to filter data sampled at apredetermined time interval (also referred to as a sample rate). In someembodiments, wherein the potentiostat is configured to measure theanalyte at discrete time intervals, these time intervals determine thesample rate of the digital filter. In some alternative embodiments,wherein the potentiostat is configured to continuously measure theanalyte, for example, using a current-to-frequency converter asdescribed above, the processor module can be programmed to request adigital value from the A/D converter at a predetermined time interval,also referred to as the acquisition time. In these alternativeembodiments, the values obtained by the processor are advantageouslyaveraged over the acquisition time due the continuity of the currentmeasurement. Accordingly, the acquisition time determines the samplerate of the digital filter.

In some embodiments, the processor module is configured to build thedata packet for transmission to an outside source, for example, an RFtransmission to a receiver. Generally, the data packet comprises aplurality of bits that can include a preamble, a unique identifieridentifying the electronics unit, the receiver, or both, (e.g. sensor IDcode), data (e.g. raw data, filtered data, or an integrated value) orerror detection or correction. Preferably, the data (transmission)packet has a length of from about 8 bits to about 128 bits, preferablyabout 48 bits; however, larger or smaller packets can be desirable incertain embodiments. The processor module can be configured to transmitany combination of raw or filtered data. In one exemplary embodiment,the transmission packet contains a fixed preamble, a unique ID of theelectronics unit, a single five-minute average (e.g. integrated) sensordata value, and a cyclic redundancy code (CRC).

In some embodiments, the processor further performs the processing, suchas storing data, analyzing data streams, calibrating analyte sensordata, estimating analyte values, comparing estimated analyte values withtime corresponding measured analyte values, analyzing a variation ofestimated analyte values, downloading data, and controlling the userinterface by providing analyte values, prompts, messages, warnings,alarms, and the like. In such cases, the processor includes hardware andsoftware that performs the processing described herein, for exampleflash memory provides permanent or semi-permanent storage of data,storing data such as sensor ID, receiver ID, and programming to processdata streams (for example, programming for performing estimation andother algorithms described elsewhere herein) and random access memory(RAM) stores the system's cache memory and is helpful in dataprocessing. Alternatively, some portion of the data processing (such asdescribed with reference to the processor elsewhere herein) can beaccomplished at another (e.g. remote) processor and can be configured tobe in wired or wireless connection therewith.

In some embodiments, an output module, which is integral with oroperatively connected with the processor, includes programming forgenerating output based on the data stream received from the sensorsystem and it's processing incurred in the processor. In someembodiments, output is generated via a user interface.

Noise

Generally, implantable sensors measure a signal related to an analyte ofinterest in a host. For example, an electrochemical sensor can measureglucose, creatinine, or urea in a host, such as an animal (e.g. ahuman). Generally, the signal is converted mathematically to a numericvalue indicative of analyte status, such as analyte concentration, asdescribed in more detail elsewhere herein. In general, the signalgenerated by conventional analyte sensors contains some noise. Noise isclinically important because it can induce error and can reduce sensorperformance, such as by providing a signal that causes the analyteconcentration to appear higher or lower than the actual analyteconcentration. For example, upward or high noise (e.g. noise that causesthe signal to increase) can cause the reading of the host's glucoseconcentration to appear higher than the actual value, which in turn canlead to improper treatment decisions. Similarly, downward or low noise(e.g. noise that causes the signal to decrease) can cause the reading ofthe host's glucose concentration to appear lower than its actual value,which in turn can also lead to improper treatment decisions.Accordingly, noise reduction is desirable.

In general, the signal detected by the sensor can be broken down intoits component parts. For example, in an enzymatic electrochemicalanalyte sensor, preferably after sensor break-in is complete, the totalsignal can be divided into an ‘analyte component,’ which isrepresentative of analyte (e.g., glucose) concentration, and a ‘noisecomponent,’ which is caused by non-analyte-related species that have aredox potential that substantially overlaps with the redox potential ofthe analyte (or measured species, e.g. H₂O₂) at an applied voltage. Thenoise component can be further divided into its component parts, e.g.constant and non-constant noise. It is not unusual for a sensor toexperience a certain level of noise. In general, ‘constant noise’(sometimes referred to as constant background or baseline) is caused bynon-analyte-related factors that are relatively stable over time,including but not limited to electroactive species that arise fromgenerally constant (e.g. daily) metabolic processes. Constant noise canvary widely between hosts. In contrast, ‘non-constant noise’ (sometimesreferred to as non-constant background) is generally caused bynon-constant, non-analyte-related species (e.g. non-constantnoise-causing electroactive species) that may arise during transientevents, such as during host metabolic processes (e.g. wound healing orin response to an illness), or due to ingestion of certain compounds(e.g. certain drugs). In some circumstances, noise can be caused by avariety of noise-causing electroactive species, which are discussed indetail elsewhere herein.

FIG. 3 is a graph illustrating the components of a signal measured by atranscutaneous glucose sensor (after sensor break-in was complete), in anon-diabetic volunteer host. The Y-axis indicates the signal amplitude(in counts) detected by the sensor. The total signal collected by thesensor is represented by line 1000, which includes components related toglucose, constant noise, and non-constant noise, which are described inmore detail elsewhere herein. In some embodiments, the total signal is araw data stream, which can include an averaged or integrated signal, forexample, using a charge-counting device.

The non-constant noise component of the total signal is represented byline 1010. The non-constant noise component 1010 of the total signal1000 can be obtained by filtering the total signal 1000 to obtain afiltered signal 1020 using any of a variety of known filteringtechniques, and then subtracting the filtered signal 1020 from the totalsignal 1000. In some embodiments, the total signal can be filtered usinglinear regression analysis of the n (e.g. 10) most recent sampled sensorvalues. In some embodiments, the total signal can be filtered usingnon-linear regression. In some embodiments, the total signal can befiltered using a trimmed regression, which is a linear regression of atrimmed mean (e.g., after rejecting wide excursions of any point fromthe regression line). In this embodiment, after the sensor recordsglucose measurements at a predetermined sampling rate (e.g., every 30seconds), the sensor calculates a trimmed mean (e.g., removes highestand lowest measurements from a data set) and then regresses theremaining measurements to estimate the glucose value. In someembodiments, the total signal can be filtered using a non-recursivefilter, such as a finite impulse response (FIR) filter. An FIR filter isa digital signal filter, in which every sample of output is the weightedsum of past and current samples of input, using only some finite numberof past samples. In some embodiments, the total signal can be filteredusing a recursive filter, such as an infinite impulse response (IIR)filter. An IIR filter is a type of digital signal filter, in which everysample of output is the weighted sum of past and current samples ofinput. In some embodiments, the total signal can be filtered using amaximum-average (max-average) filtering algorithm, which smoothes databased on the discovery that the substantial majority of signal artifactsobserved after implantation of glucose sensors in humans, for example,is not distributed evenly above and below the actual blood glucoselevels. It has been observed that many data sets are actuallycharacterized by extended periods in which the noise appears to trenddownwardly from maximum values with occasional high spikes. To overcomethese downward trending signal artifacts, the max-average calculationtracks with the highest sensor values, and discards the bulk of thelower values. Additionally, the max-average method is designed to reducethe contamination of the data with unphysiologically high data from thehigh spikes. The max-average calculation smoothes data at a samplinginterval (e.g. every 30 seconds) for transmission to the receiver at aless frequent transmission interval (e.g. every 5 minutes), to minimizethe effects of low non-physiological data. First, the microprocessorfinds and stores a maximum sensor counts value in a first set of sampleddata points (e.g. 5 consecutive, accepted, thirty-second data points). Aframe shift time window finds a maximum sensor counts value for each setof sampled data (e.g. each 5-point cycle length) and stores each maximumvalue. The microprocessor then computes a rolling average (e.g. 5-pointaverage) of these maxima for each sampling interval (e.g. every 30seconds) and stores these data. Periodically (e.g. every 10^(th)interval), the sensor outputs to the receiver the current maximum of therolling average (e.g. over the last 10 thirty-second intervals as asmoothed value for that time period (e.g. 5 minutes). In someembodiments, the total signal can be filtered using a ‘Cone ofPossibility Replacement Method,’ which utilizes physiologicalinformation along with glucose signal values in order define a ‘cone’ ofphysiologically feasible glucose signal values within a human.Particularly, physiological information depends upon the physiologicalparameters obtained from continuous studies in the literature as well asour own observations. A first physiological parameter uses a maximalsustained rate of change of glucose in humans (e.g. about 4 to 6mg/dl/min) and a maximum sustained acceleration of that rate of change(e.g. about 0.1 to 0.2 mg/min/min). A second physiological parameteruses the knowledge that rate of change of glucose is lowest at themaxima and minima, which are the areas of greatest risk in patienttreatment. A third physiological parameter uses the fact that the bestsolution for the shape of the curve at any point along the curve over acertain time period (e.g. about 20-25 minutes) is a straight line. It isnoted that the maximum rate of change can be narrowed in some instances.Therefore, additional physiological data can be used to modify thelimits imposed upon the Cone of Possibility Replacement Method forsensor glucose values. For example, the maximum per minute rate ofchange can be lower when the subject is lying down or sleeping; on theother hand, the maximum per minute rate change can be higher when thesubject is exercising, for example. In some embodiments, the totalsignal can be filtered using reference changes in electrode potential toestimate glucose sensor data during positive detection of signalartifacts from an electrochemical glucose sensor, the method hereinafterreferred to as reference drift replacement; in this embodiment, theelectrochemical glucose sensor comprises working, counter, and referenceelectrodes. This method exploits the function of the reference electrodeas it drifts to compensate for counter electrode limitations duringoxygen deficits, pH changes, or temperature changes. In alternativeimplementations of the reference drift method, a variety of algorithmscan therefore be implemented based on the changes measured in thereference electrode. Linear algorithms, and the like, are suitable forinterpreting the direct relationship between reference electrode driftand the non-glucose rate limiting signal noise such that appropriateconversion to signal noise compensation can be derived. Additionaldescription of signal filtering can be found in U.S. Patent PublicationNo. US-2005-0043598-A1.

The constant noise signal component 1030 can be obtained by calibratingthe sensor signal using reference data, such as one or more bloodglucose values obtained from a hand-held blood glucose meter, or thelike, from which the baseline ‘b’ of a regression can be obtained,representing the constant noise signal component 1030.

The analyte signal component 1040 can be obtained by subtracting theconstant noise signal component 1030 from the filtered signal 1020.

In general, non-constant noise is caused by interfering species(non-constant noise-causing species), which can be compounds, such asdrugs that have been administered to the host, or intermittentlyproduced products of various host metabolic processes. Exemplaryinterferents include but are not limited to a variety of drugs (e.g.acetaminophen), H₂O2 from exterior sources (e.g. produced outside thesensor membrane system), and reactive metabolic species (e.g. reactiveoxygen and nitrogen species, some hormones, etc.). Some knowninterfering species for a glucose sensor include but are not limited toacetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine,dopamine, ephedrine, ibuprofen, L-dopa, methyldopa, salicylate,tetracycline, tolazamide, tolbutamide, triglycerides, and uric acid. Ithas also been observed that non-constant noise may increase when a hostis intermittently sedentary, such as when sleeping or sitting forextended periods. This noise may dissipate upon renewed activity by thehost. Additional description of this effect can be found in U.S. PatentPublication No. US-2009-0247856-A1.

Interferents

Interferents are molecules or other species that may cause a sensor togenerate a false positive or negative analyte signal (e.g. anon-analyte-related signal). Some interferents are known to becomereduced or oxidized at the electrochemically reactive surfaces of thesensor, while other interferents are known to interfere with the abilityof the enzyme (e.g. glucose oxidase) used to react with the analytebeing measured. Yet other interferents are known to react with theenzyme (e.g. glucose oxidase) to produce a byproduct that iselectrochemically active. Interferents can exaggerate or mask theresponse signal, thereby leading to false or misleading results. Forexample, a false positive signal may cause the host's analyteconcentration (e.g., glucose concentration) to appear higher than thetrue analyte concentration. False-positive signals may pose a clinicallysignificant problem in some conventional sensors. For example in asevere hypoglycemic situation, in which the host has ingested aninterferent (e.g. acetaminophen), the resulting artificially highglucose signal can lead the host to believe that he is euglycemic orhyperglycemic. In response, the host may make inappropriate treatmentdecisions, such as by injecting himself with too much insulin, or bytaking no action, when the proper course of action would be to begineating. In turn, this inappropriate action or inaction may lead to adangerous hypoglycemic episode for the host. Accordingly, certainembodiments contemplated herein include a membrane system thatsubstantially reduces or eliminates the effects of interferents onanalyte measurements. These membrane systems may include one or moredomains capable of blocking or substantially reducing the flow ofinterferents onto the electroactive surfaces of the electrode may reducenoise and improve sensor accuracy as described in more detail in U.S.Patent Publication No. US-2009-0247856-A1.

Membrane System with Improved Low Oxygen Performance

Performance of enzymatic glucose sensors which rely on oxygen as anelectron mediator is significantly affected by the amount of oxygenpresent at the electrode surface. In these sensors, oxygen acts as aco-substrate and reduction of oxygen to hydrogen peroxide is measured asan indication of glucose levels. However, in order to obtain an accuratemeasure of the glucose, oxygen cannot be the limiting reactant. It hasbeen observed that under low oxygen conditions (such as at about 0.25mg/L or lower) signal drift becomes a significant problem, particularlyfor sensors with high glucose sensitivity. For example, FIG. 4 showsincreasing signal drift (i.e., a drift toward less signal) withincreasing glucose sensitivity in the presence of about 0.25 mg/Loxygen. As illustrated in FIG. 4, the low sensitivity sensors under theabove-described low oxygen conditions had negative changes in signals(about −2.5% to about −4.1%) compared to signals under ambient oxygenconditions. However, these negative changes in signal in signal (about−2.5% to about −4.1%) for the low sensitivity sensors were less severethan the negative changes in signals (about −4.5% to about −7.4%) forhigh sensitivity sensors.

Clearly, as enzymatic glucose sensors capable of increased glucosesensitivity are developed, there is a desire for improved sensorperformance under low oxygen conditions.

As discussed in greater detail below, it has been found that includingone or more hydrophilic polymers in the polymer system of the enzymedomain results in improved sensor performance (i.e., less signal drift)under low oxygen conditions.

Membrane Fabrication

Preferably, polymers of the preferred embodiments may be processed bysolution-based techniques, for example, dipping, casting,electrospinning, vapor deposition, spin coating, coating, and the like.Water-based polymer emulsions can be fabricated to form membranes bymethods similar to those used for solvent-based materials. In both casesthe evaporation of a volatile liquid (e.g. organic solvent or water)leaves behind a film of the polymer. Cross-linking of the deposited filmmay be performed through the use of multi-functional reactiveingredients by a number of methods well known to those skilled in theart. The liquid system may cure by heat, moisture, high-energyradiation, ultraviolet light, or by completing the reaction, whichproduces the final polymer in a mold or on a substrate to be coated.

However, care must be taken in the fabrication process because theenzymes used in enzymatic sensors are sensitive to temperature and pHinduced degradation. Thus, fabrication processes must be conducted underpH and temperature conditions which preserve enzymatic activity. Forexample, a process of heat curing polymer membranes after application ofan enzyme layer is limited to temperatures below that which willdenature the enzyme. Unfortunately, such process temperatures aretypically well below curing temperatures tolerable by the polymersystem. The requirement to cure polymer membranes at these restrictedtemperatures extends curing time, increasing cost and limitingthroughput in device fabrication. Accordingly, there is a desire forenzyme stabilization in enzymatic glucose sensors so as provideenhancement in the tolerable pH range and increased thermal stability ofthe enzyme in order to decrease manufacturing time and cost.

Domains that include at least two surface-active group-containingpolymers may be made using any of the methods of forming polymer blendsknown in the art. In one exemplary embodiment, a solution of apolyurethane containing silicone end groups is mixed with a solution ofa polyurethane containing fluorine end groups (e.g. wherein thesolutions include the polymer dissolved in a suitable solvent such asacetone, ethyl alcohol, DMAC, THF, 2-butanone, and the like). Themixture can then be drawn into a film or applied to a surface using anymethod known in the art (e.g. spraying, painting, dip coating, vapordepositing, molding, 3-D printing, lithographic techniques (e.g.photolithograph), micro- and nano-pipetting printing techniques, etc.).The mixture can then be cured under high temperature (e.g. 50-150° C.).Other suitable curing methods may include ultraviolet or gammaradiation, for example.

Some amount of cross-linking agent can also be included in the mixtureto induce cross-linking between polymer molecules. Non-limiting examplesof suitable cross-linking agents include isocyanate, carbodiimide,gluteraldehyde or other aldehydes, epoxy, acrylates, free-radical basedagents, ethylene glycol diglycidyl ether (EGDE), poly(ethylene glycol)diglycidyl ether (PEGDE), or dicumyl peroxide (DCP). In one embodiment,from about 0.1% to about 15% w/w of cross-linking agent is addedrelative to the total dry weights of cross-linking agent and polymersadded when blending the ingredients (in one example, about 1% to about10%). During the curing process, substantially all of the cross-linkingagent is believed to react, leaving substantially no detectableunreacted cross-linking agent in the final film.

Bioprotective Domain

The bioprotective domain is the domain or layer of an implantable deviceconfigured to interface with (e.g. contact) a biological fluid whenimplanted in a host or connected to the host (e.g. via an intravascularaccess device providing extracorporeal access to a blood vessel). Thenoise reducing capacity of certain bioprotective domains is described inmore detail in U.S. Patent Publication No. US-2009-0247856-A1.

Some embodiments described herein may include membranes which comprise abioprotective domain 248 (see FIG. 2B), also referred to as abioprotective layer, including at least one polymer containing asurface-active group. In some embodiments, the surface-activegroup-containing polymer is a surface-active end group-containingpolymer. In some of these embodiments, the surface-active endgroup-containing polymer is a polymer having covalently bondedsurface-active end groups. However, it is contemplated that othersurface-active group-containing polymers may also be used and can beformed by modification of fully-reacted base polymers via the graftingof side chain structures, surface treatments or coatings applied aftermembrane fabrication (e.g., via surface-modifying additives), blendingof a surface-modifying additive to a base polymer before membranefabrication, immobilization of the surface-active-group-containing softsegments by physical entrainment during synthesis, or the like. Certainexemplary bioprotective domains which may be used in some embodiments asdescribed herein are described in more detail in U.S. Patent PublicationNo. US-2009-0247856-A1.

Base polymers useful for certain embodiments may include any linear orbranched polymer on the backbone structure of the polymer. Suitable basepolymers may include, but are not limited to, epoxies, polyolefins,polysiloxanes, polyethers, acrylics, polyesters, carbonates, andpolyurethanes, wherein polyurethanes may include polyurethane copolymerssuch as polyether-urethane-urea, polycarbonate-urethane,polyether-urethane, silicone-polyether-urethane,silicone-polycarbonate-urethane, polyester-urethane, and the like. Insome embodiments, base polymers may be selected for their bulkproperties, such as, but not limited to, tensile strength, flex life,modulus, and the like. For example, polyurethanes are known to berelatively strong and to provide numerous reactive pathways, whichproperties may be advantageous as bulk properties for a membrane domainof the continuous sensor.

In some embodiments, a base polymer synthesized to have hydrophilicsegments may be used to form the bioprotective layer. For example, alinear base polymer including biocompatible segmented block polyurethanecopolymers comprising hard and soft segments may be used. In someembodiments, the hard segment of the copolymer may have a molecularweight of from about 160 daltons to about 10,000 daltons, and sometimesfrom about 200 daltons to about 2,000 daltons. In some embodiments, themolecular weight of the soft segment may be from about 200 daltons toabout 10,000,000 daltons, and sometimes from about 500 daltons to about5,000,000 daltons, and sometimes from about 500,00 daltons to about2,000,000 daltons. It is contemplated that polyisocyanates used for thepreparation of the hard segments of the copolymer may be aromatic oraliphatic diisocyanates. The soft segments used in the preparation ofthe polyurethane may be a polyfunctional aliphatic polyol, apolyfunctional aliphatic or aromatic amine, or the like that may beuseful for creating permeability of the analyte (e.g. glucose)therethrough, and may include, for example, polyvinyl acetate (PVA),poly(ethylene glycol) (PEG), polyacrylamide, acetates, polyethyleneoxide (PEO), polyethylacrylate (PEA), polyvinylpyrrolidone (PVP), andvariations thereof (e.g. PVP vinyl acetate), and wherein PVP andvariations thereof may be preferred for their hydrolytic stability insome embodiments.

Alternatively, in some embodiments, the bioprotective layer may comprisea combination of a base polymer (e.g. polyurethane) and one or morehydrophilic polymers, such as, PVA, PEG, polyacrylamide, acetates, PEO,PEA, PVP, and variations thereof (e.g. PVP vinyl acetate), e.g. as aphysical blend or admixture wherein each polymer maintains its uniquechemical nature. It is contemplated that any of a variety of combinationof polymers may be used to yield a blend with desired glucose, oxygen,and interference permeability properties. For example, in someembodiments, the bioprotective layer may be formed from a blend of apolycarbonate-urethane base polymer and PVP, but in other embodiments, ablend of a polyurethane, or another base polymer, and one or morehydrophilic polymers may be used instead. In some of the embodimentsinvolving use of PVP, the PVP portion of the polymer blend may comprisefrom about 5% to about 50% by weight of the polymer blend, sometimesfrom about 15% to 20%, and other times from about 25% to 40%. It iscontemplated that PVP of various molecular weights may be used. Forexample, in some embodiments, the molecular weight of the PVP used maybe from about 25,000 daltons to about 5,000,000 daltons, sometimes fromabout 50,000 daltons to about 2,000,000 daltons, and other times from6,000,000 daltons to about 10,000,000 daltons.

The term ‘surface-active group’ and ‘surface-active end group’ as usedherein are broad terms and are used in their ordinary sense, including,without limitation, surface-active oligomers or other surface-activemoieties having surface-active properties, such as alkyl groups, whichpreferentially migrate towards a surface of a membrane formed therefrom. Surface-active groups preferentially migrate toward air (e.g.,driven by thermodynamic properties during membrane formation). In someembodiments, the surface-active groups are covalently bonded to the basepolymer during synthesis. In some preferred embodiments, surface-activegroups may include silicone, sulfonate, fluorine, polyethylene oxide,hydrocarbon groups, and the like. The surface activity (e.g., chemistry,properties) of a membrane domain including a surface-activegroup-containing polymer reflects the surface activity of thesurface-active groups rather than that of the base polymer. In otherwords, surface-active groups control the chemistry at the surface (e.g.,the biological contacting surface) of the membrane without compromisingthe bulk properties of the base polymer. The surface-active groups ofthe preferred embodiments are selected for desirable surface properties,for example, non-constant noise-blocking ability, break-in time(reduced), ability to repel charged species, cationic or anionicblocking, or the like. In some preferred embodiments, the surface-activegroups are located on one or more ends of the polymer backbone, andreferred to as surface-active end groups, wherein the surface-active endgroups are believed to more readily migrate to the surface of thebioprotective domain/layer formed from the surface-activegroup-containing polymer in some circumstances.

In some embodiments, the bioprotective domain 248 is formed from apolymer containing silicone as the surface-active group, for example, apolyurethane containing silicone end group(s). Some embodiments includea continuous analyte sensor configured for insertion into a host,wherein the sensor has a membrane located over the sensing mechanism,which includes a polyurethane comprising silicone end groups configuredto substantially block the effect of non-constant noise-causing specieson the sensor signal, as described in more detail elsewhere herein. Insome embodiments, the polymer includes about 10%, 11%, 12%, 13%, 14%,15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%,29%, 30%, to about 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%,41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54% or55% silicone by weight. In certain embodiments, the silicone (e.g. aprecursor such as PDMS) has a molecular weight from about 500 to about10,000 daltons, preferably at least about 200 daltons. In someembodiments, the base polymer includes at least about 10% silicone byweight, and preferably from about 19% to about 40% silicone by weight.These ranges are described in U.S. Patent Publication No.US-2009-0247856-A1 as providing an advantageous balance ofnoise-reducing functionality, while maintaining sufficient glucosepermeability in embodiments wherein the sensor is a glucose sensor, forexample.

In some embodiments, the bioprotective domain is formed from a polymercontaining fluorine as a surface-active group, for example, apolyurethane that contains a fluorine end groups. In preferredembodiments, the polymer includes from about 1% to about 25% fluorine byweight. Some embodiments include a continuous analyte sensor configuredfor insertion into a host, wherein the sensor has a membrane locatedover the sensing mechanism, wherein the membrane includes a polyurethanecontaining fluorine surface-active groups, and wherein the membrane isconfigured and arranged to reduce a break-in time of a sensor ascompared to a membrane formed from a similar base polymer without thesurface-active group(s). For example, in preferred embodiments, aglucose sensor having a bioprotective domain of the preferredembodiments has a response time (e.g. t.sub.90) of less than 120seconds, sometimes less than 60 seconds, and sometimes less than about45, 30, 20, or 10 seconds (across a physiological range of glucoseconcentration).

In some embodiments, the bioprotective domain may be formed from apolymer that contains sulfonate as a surface-active group, for example,a polyurethane containing sulfonate end group(s). In some embodiments,the continuous analyte sensor configured for insertion into a host mayinclude a membrane located over the sensing mechanism, wherein themembrane includes a polymer that contains sulfonate as a surface-activegroup, and is configured to repel charged species, for example, due tothe net negative charge of the sulfonated groups.

In some embodiments, a blend of two or more (e.g. two, three, four,five, or more) surface-active group-containing polymers is used to forma bioprotective membrane domain. For example, by blending a polyurethanewith silicone end groups and a polyurethane with fluorine end groups,and forming a bioprotective membrane domain from that blend, a sensorcan be configured to substantially block non-constant noise-causingspecies and reduce the sensor's t₉₀, as described in more detailelsewhere herein. Similarly, by blending a polyurethane containingsilicone end groups, a polyurethane containing fluorine end groups, anda polyurethane containing sulfonate end groups, and forming abioprotective membrane domain from that blend, a sensor can beconfigured to substantially block non-constant noise-causing species, toreduce the sensor's break-in time and to repel charged species, asdescribed in more detail above. Although in some embodiments, blendingof two or more surface-active group-containing polymers is used, inother embodiments, a single component polymer can be formed bysynthesizing two or more surface-active groups with a base polymer toachieve similarly advantageous surface properties; however, blending maybe preferred in some embodiments for ease of manufacture.

In some embodiments, the bioprotective domain 248 is positioned mostdistally to the sensing region such that its outer most domain contactsa biological fluid when inserted in vivo. In some embodiments, thebioprotective domain is resistant to cellular attachment, impermeable tocells, and may be composed of a biostable material. While not wishing tobe bound by theory, it is believed that when the bioprotective domain248 is resistant to cellular attachment (for example, attachment byinflammatory cells, such as macrophages, which are therefore kept asufficient distance from other domains, for example, the enzyme domain),hypochlorite and other oxidizing species are short-lived chemicalspecies in vivo and biodegradation does not generally occur.Additionally, the materials preferred for forming the bioprotectivedomain 248 may be resistant to the effects of these oxidative speciesand have thus been termed biodurable. In some embodiments, thebioprotective domain controls the flux of oxygen and other analytes (forexample, glucose) to the underlying enzyme domain (e.g. wherein thefunctionality of the diffusion resistance domain is built-into thebioprotective domain such that a separate diffusion resistance domain isnot required).

In certain embodiments, the thickness of the bioprotective domain may befrom about 0.1, 0.5, 1, 2, 4, 6, 8 microns or less to about 10, 15, 20,30, 40, 50, 75, 100, 125, 150, 175, 200 or 250 microns or more. In someof these embodiments, the thickness of the bioprotective domain may besometimes from about 1 to about 5 microns, and sometimes from about 2 toabout 7 microns. In other embodiments, the bioprotective domain may befrom about 20 or 25 microns to about 50, 55, or 60 microns thick. Insome embodiments, the glucose sensor may be configured fortranscutaneous or short-term subcutaneous implantation, and may have athickness from about 0.5 microns to about 8 microns, and sometimes fromabout 4 microns to about 6 microns. In one glucose sensor configured forfluid communication with a host's circulatory system, the thickness maybe from about 1.5 microns to about 25 microns, and sometimes from about3 to about 15 microns. It is also contemplated that in some embodiments,the bioprotective layer or any other layer of the electrode may have athickness that is consistent, but in other embodiments, the thicknessmay vary. For example, in some embodiments, the thickness of thebioprotective layer may vary along the longitudinal axis of theelectrode end.

Diffusion Resistance Domain

In some embodiments, a diffusion resistance domain 246, also referred toas a diffusion resistance layer, may be used and is situated moreproximal to the implantable device relative to the bioprotective domain.In some embodiments, the functionality of the diffusion resistancedomain may be built into the bioprotective domain that comprises thesurface-active group-containing base polymer. Accordingly, it is to benoted that the description herein of the diffusion resistance domain mayalso apply to the bioprotective domain. The diffusion resistance domainserves to control the flux of oxygen and other analytes (for example,glucose) to the underlying enzyme domain. As described in more detailelsewhere herein, there exists a molar excess of glucose relative to theamount of oxygen in blood, i.e., for every free oxygen molecule inextracellular fluid, there are typically more than 100 glucose moleculespresent (see Updike et al., Diabetes Care 5:207-21 (1982)). However, animmobilized enzyme-based sensor employing oxygen as cofactor is suppliedwith oxygen in non-rate-limiting excess in order to respond linearly tochanges in glucose concentration, while not responding to changes inoxygen tension. More specifically, when a glucose-monitoring reaction isoxygen-limited, linearity is not achieved above minimal concentrationsof glucose. Without a semipermeable membrane situated over the enzymedomain to control the flux of glucose and oxygen, a linear response toglucose levels can be obtained only up to about 40 mg/dL. However, in aclinical setting, a linear response to glucose levels is desirable up toat least about 500 mg/dL.

The diffusion resistance domain 246 includes a semipermeable membranethat controls the flux of oxygen and glucose to the underlying enzymedomain 244, preferably rendering oxygen in non-rate-limiting excess. Asa result, the upper limit of linearity of glucose measurement isextended to a much higher value than that which is achieved without thediffusion resistance domain. In some embodiments, the diffusionresistance domain exhibits an oxygen-to-glucose permeability ratio ofapproximately 200:1, but in other embodiments the oxygen-to-glucosepermeability ratio may be approximately 100:1, 125:1, 130:1, 135:1,150:1, 175:1, 225:1, 250:1, 275:1, 300:1, or 500:1. As a result of thehigh oxygen-to-glucose permeability ratio, one-dimensional reactantdiffusion may provide sufficient excess oxygen at all reasonable glucoseand oxygen concentrations found in the subcutaneous matrix (See Rhodeset al., Anal. Chem., 66:1520-1529 (1994)). In some embodiments, a lowerratio of oxygen-to-glucose can be sufficient to provide excess oxygen byusing a high oxygen soluble domain (for example, a silicone material) toenhance the supply/transport of oxygen to the enzyme membrane orelectroactive surfaces. By enhancing the oxygen supply through the useof a silicone composition, for example, glucose concentration can beless of a limiting factor. In other words, if more oxygen is supplied tothe enzyme or electroactive surfaces, then more glucose can also besupplied to the enzyme without creating an oxygen rate-limiting excess.

In some embodiments, the diffusion resistance domain is formed of a basepolymer synthesized to include a polyurethane membrane with bothhydrophilic and hydrophobic regions to control the diffusion of glucoseand oxygen to an analyte sensor. A suitable hydrophobic polymercomponent may be a polyurethane or polyether urethane urea. Polyurethaneis a polymer produced by the condensation reaction of a diisocyanate anda difunctional hydroxyl-containing material. A polyurea is a polymerproduced by the condensation reaction of a diisocyanate and adifunctional amine-containing material. Preferred diisocyanates includealiphatic diisocyanates containing from about 4 to about 8 methyleneunits. Diisocyanates containing cycloaliphatic moieties can also beuseful in the preparation of the polymer and copolymer components of themembranes of preferred embodiments. The material that forms the basis ofthe hydrophobic matrix of the diffusion resistance domain can be any ofthose known in the art as appropriate for use as membranes in sensordevices and as having sufficient permeability to allow relevantcompounds to pass through it, for example, to allow an oxygen moleculeto pass through the membrane from the sample under examination in orderto reach the active enzyme or electrochemical electrodes. Examples ofmaterials which can be used to make non-polyurethane type membranesinclude vinyl polymers, polyethers, polyesters, polyamides, inorganicpolymers such as polysiloxanes and polycarbosiloxanes, natural polymerssuch as cellulosic and protein based materials, and mixtures orcombinations thereof.

In one embodiment of a polyurethane-based resistance domain, thehydrophilic polymer component is polyethylene oxide. For example, oneuseful hydrophilic copolymer component is a polyurethane polymer thatincludes about 20% hydrophilic polyethylene oxide. The polyethyleneoxide portions of the copolymer are thermodynamically driven to separatefrom the hydrophobic portions of the copolymer and the hydrophobicpolymer component. The 20% polyethylene oxide-based soft segment portionof the copolymer used to form the final blend affects the water pick-upand subsequent glucose permeability of the membrane.

Alternatively, in some embodiments, the resistance domain may comprise acombination of a base polymer (e.g. polyurethane) and one or morehydrophilic polymers (e.g. PVA, PEG, polyacrylamide, acetates, PEO, PEA,PVP, and variations thereof). It is contemplated that any of a varietyof combination of polymers may be used to yield a blend with desiredglucose, oxygen, and interference permeability properties. For example,in some embodiments, the resistance domain may be formed from a blend ofa silicone polycarbonate-urethane base polymer and a PVP hydrophilicpolymer, but in other embodiments, a blend of a polyurethane, or anotherbase polymer, and one or more hydrophilic polymers may be used instead.In some of the embodiments involving the use of PVP, the PVP portion ofthe polymer blend may comprise from about 5% to about 50% by weight ofthe polymer blend, sometimes from about 15% to 20%, and other times fromabout 25% to 40%. It is contemplated that PVP of various molecularweights may be used. For example, in some embodiments, the molecularweight of the PVP used may be from about 25,000 daltons to about5,000,000 daltons, sometimes from about 50,000 daltons to about2,000,000 daltons, and other times from 6,000,000 daltons to about10,000,000 daltons.

In some embodiments, the diffusion resistance domain 246 can be formedas a unitary structure with the bioprotective domain 248; that is, theinherent properties of the diffusion resistance domain 246 areincorporated into bioprotective domain 248 such that the bioprotectivedomain 248 functions as a diffusion resistance domain 246.

In certain embodiments, the thickness of the resistance domain may befrom about 0.05 microns or less to about 200 microns or more. In some ofthese embodiments, the thickness of the resistance domain may be fromabout 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2,2.5, 3, 3.5, 4, 6, 8 microns to about 9, 10, 11, 12, 13, 14, 15, 16, 17,18, 19, 19.5, 20, 30, 40, 50, 60, 70, 75, 80, 85, 90, 95, or 100microns. In some embodiments, the thickness of the resistance domain isfrom about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns in thecase of a transcutaneously implanted sensor or from about 20 or 25microns to about 40 or 50 microns in the case of a wholly implantedsensor.

Enzyme Domain for Improved Low Oxygen Performance

In some embodiments, an enzyme domain 244, also referred to as theenzyme layer, may be used and is situated less distal from theelectrochemically reactive surfaces than the diffusion resistance domain246 and/or the bioprotective domain 248. The enzyme domain comprises acatalyst configured to react with an analyte. In one embodiment, theenzyme domain is an immobilized enzyme domain 244 including glucoseoxidase. In other embodiments, the enzyme domain 244 can be impregnatedwith other oxidases, for example, galactose oxidase, cholesteroloxidase, amino acid oxidase, alcohol oxidase, lactate oxidase, oruricase. For example, for an enzyme-based electrochemical glucose sensorto perform well, the sensor's response should neither be limited byenzyme activity nor cofactor concentration.

In some embodiments, the catalyst (enzyme) can be impregnated orotherwise immobilized into the bioprotective and/or diffusion resistancedomain such that a separate enzyme domain 244 is not required (e.g.wherein a unitary domain is provided including the functionality of thebioprotective domain, diffusion resistance domain, and enzyme domain).In some embodiments, the enzyme domain 244 is formed at least in partfrom polyurethane, for example, aqueous dispersions of colloidalpolyurethane polymers including the enzyme.

Through experiments, it has been unexpectedly found that the use of PVPblended with a base polymer, such as a polyurethane, may provide theenzyme domain with the capability of substantially improving sensorperformance in low oxygen conditions (e.g., such as at oxygenconcentration of about 0.25 mg/L or less). This increase in sensorperformance allows for the achievement of increased glucose sensitivity(e.g., from 5 to 200 pA/mg/dL) under low oxygen conditions. Although PVPis described here to provide an example of a hydrophilic polymer capableof providing this effect, it is contemplated that any of a variety ofother hydrophilic polymers may also be used, such as, polyvinylpyrrolidone-vinyl acetate (PVP-VA), hydroxypropyl cellulose (HPC),hydroxypropyl methylcellulose (HPMC), for example. Thus, in someembodiments, the enzyme domain comprises a curable mixture of a urethanepolymer and a hydrophilic polymer.

In some embodiments, the hydrophilic portion of the polymer blend (e.g.,PVP) may comprise from about 5% to about 50% by weight of the polymerblend, such as from about 5% to 30%, such as from about 10% to 25%, suchas from about 15% to 25%, such as about 15%. In embodiments where PVP isthe hydrophilic polymer, it is contemplated that PVP of variousmolecular weights may be used. For example, in some embodiments, themolecular weight of the PVP used may be from about 25,000 daltons toabout 5,000,000 daltons, sometimes from about 50,000 daltons to about2,000,000 daltons, and other times from 6,000,000 daltons to about10,000,000 daltons.

It is contemplated that in certain embodiments, the base polymer andhydrophilic polymer blend may not be crosslinked, but in otherembodiments, crosslinking may be used and achieved by any of a varietyof methods, for example, by adding a crosslinking agent. In someembodiments, a polyurethane polymer may be crosslinked in the presenceof PVP by preparing a premix of the polymers and adding a cross-linkingagent just prior to the production of the membrane. Suitablecross-linking agents contemplated include, but are not limited to,carbodiimides (e.g. 1-ethyl-3-(3-dimethylaminopropyl) carbodiimidehydrochloride, and UCARLNK® XL-25 (Union Carbide)), epoxides andmelamine/formaldehyde resins. Alternatively, it is also contemplatedthat crosslinking may be achieved by irradiation at a wavelengthsufficient to promote crosslinking between the hydrophilic polymermolecules, which is believed to create a more tortuous diffusion paththrough the domain.

In some embodiments, the thickness of the enzyme domain may be fromabout 0.01, 0.05, 0.6, 0.7, or 0.8 microns to about 1, 1.2, 1.4, 1.5,1.6, 1.8, 2, 2.1, 2.2, 2.5, 3, 4, 5, 10, 20, 30 40, 50, 60, 70, 80, 90,or 100 microns. In more preferred embodiments, the thickness of theenzyme domain is between about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35,0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, 4, or 5 microns and 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, 19.5, 20, 25, or 30 microns. In evenmore preferred embodiments, the thickness of the enzyme domain is fromabout 2, 2.5, or 3 microns to about 3.5, 4, 4.5, or 5 microns in thecase of a transcutaneously implanted sensor or from about 6, 7, or 8microns to about 9, 10, 11, or 12 microns in the case of a whollyimplanted sensor.

Enzyme Domain with Improved Enzyme Stability

As described above, an enzyme domain 244 comprises a catalyst configuredto react with an analyte. In some embodiments, catalysts included in anenzyme domain are susceptible to thermal or pH induced degradation. Insome related embodiments, the enzyme domain may also comprise one ormore enzyme stabilizing agents. Such agents improve the enzyme's abilityto resist thermal or pH induced denaturing. Inclusion of enzymestabilizing agents thus facilitates device fabrication by allowing foruse of fabrication processes which would otherwise compromise theenzyme's activity. Inclusion of these agents has the added benefits ofextending useable and shelf lives of the sensors.

Any material which improves thermal and/or pH stability of the enzyme,without affecting analyte or oxygen permeability of the enzyme domain tothe point that the enzyme domain is no longer suitable for use in asensor, may be used as an enzyme stabilizing agent. In some embodiments,an enzyme stabilizing agent may be dipolar. Without wishing to be boundby theory, it is believed that dipolar enzyme stabilizing agentsstabilize the enzyme by orienting around the enzyme in such a way as toprovide a charged local environment that stabilizes the enzyme'stertiary structure.

Dipolar enzyme stabilizing agents may be zwitterionic ornon-zwitterionic. That is, dipolar enzyme stabilizing agents are neutralmolecules with a positive and negative electrical charge at differentlocations. In some embodiments, the positive and negative electricalcharges are full unit charges (i.e., the molecules are zwitterionic). Inother embodiments, the positive and negative charges are less than fullunit charges (i.e., the molecules are dipolar, but non-zwitterionic).

In some embodiments, a zwitterionic enzyme stabilizing agent may be abetaine, such as glycine betaine, poly(carboxybetaine) (pCB), orpoly(sulfobetaine) (pSB), or some other zwitterion, such as ectoine orhydroxyectoine. In preferred embodiments, the zwitterionic enzymestabilizing agent is glycine betaine. In some embodiments, anon-zwitterionic enzyme stabilizing agent may be an amine oxide.

In embodiments where the enzyme domain comprises an enzyme stabilizingreagent, the amount of enzyme stabilizing reagent present in the enzymedomain is sufficient to provide an improvement in the thermal and/or pHstability of the enzyme, while not disrupting the permeabilitycharacteristics of the enzyme layer so that the sensor retains highglucose sensitivity. The identity and amount of enzyme stabilizingreagent used in the enzyme domain may vary based on the particularenzyme used in the sensor; however, the amount of enzyme stabilizingreagent is generally less than about 50% wt. of the amount of theenzyme; such as less than about 25% wt; such as less than about 10 wt.%. In a preferred embodiment, the enzyme is glucose oxidase and theenzyme stabilizing reagent is a betaine, such as glycine betaine.

As described above for enzyme domains with improved low oxygenperformance, the enzyme and enzyme stabilizing reagent can beimpregnated or otherwise immobilized into the bioprotective or diffusionresistance domain such that a separate enzyme domain 244 is not required(e.g. wherein a unitary domain is provided including the functionalityof the bioprotective domain, diffusion resistance domain, and enzymedomain). In some embodiments, the enzyme domain 244 is formed from apolyurethane, for example, aqueous dispersions of colloidal polyurethanepolymers including the enzyme and enzyme stabilizing reagent. Again, itis contemplated that in some embodiments, the polymer system of theenzyme domain may not be crosslinked, but in other embodiments,crosslinking may be used and achieved by any of a variety of methods,for example, by adding a crosslinking agent.

In some embodiments, the thickness of the enzyme domain may be fromabout 0.01, 0.05, 0.6, 0.7, or 0.8 microns to about 1, 1.2, 1.4, 1.5,1.6, 1.8, 2, 2.1, 2.2, 2.5, 3, 4, 5, 10, 20, 30 40, 50, 60, 70, 80, 90,or 100 microns. In more preferred embodiments, the thickness of theenzyme domain is between about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35,0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, 4, or 5 microns and 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, 19.5, 20, 25, or 30 microns. In evenmore preferred embodiments, the thickness of the enzyme domain is fromabout 2, 2.5, or 3 microns to about 3.5, 4, 4.5, or 5 microns in thecase of a transcutaneously implanted sensor or from about 6, 7, or 8microns to about 9, 10, 11, or 12 microns in the case of a whollyimplanted sensor.

Finally, in some embodiments, enzyme stabilizing agents may be includedin the enzyme domains with improved low oxygen performance describedabove. In other words, in some embodiments, the enzyme domain comprisesan enzyme, a base polymer, a hydrophilic polymer, and an enzymestabilizing agent.

Interference Domain

It is contemplated that in some embodiments, such as the embodimentillustrated in FIGS. 2A-2B, an optional interference domain 242, alsoreferred to as the interference layer, may be provided, in addition tothe bioprotective domain and the enzyme domain. The interference domain242 may substantially reduce the permeation of one or more interferentsinto the electrochemically reactive surfaces. Preferably, theinterference domain 242 f is configured to be much less permeable to oneor more of the interferents than to the measured species. It is alsocontemplated that in some embodiments, where interferent blocking may beprovided by the bioprotective domain (e.g. via a surface-activegroup-containing polymer of the bioprotective domain), a separateinterference domain may not be used.

In some embodiments, the interference domain is formed from asilicone-containing polymer, such as a polyurethane containing silicone,or a silicone polymer. While not wishing to be bound by theory, it isbelieved that, in order for an enzyme-based glucose sensor to functionproperly, glucose would not have to permeate the interference layer,where the interference domain is located more proximal to theelectroactive surfaces than the enzyme domain. Accordingly, in someembodiments, a silicone-containing interference domain, comprising agreater percentage of silicone by weight than the bioprotective domain,may be used without substantially affecting glucose concentrationmeasurements. For example, in some embodiments, the silicone-containinginterference domain may comprise a polymer with a high percentage ofsilicone (e.g. from about 25%, 30%, 35%, 40%, 45%, or 50% to about 60%,70%, 80%, 90% or 95%).

In one embodiment, the interference domain may include ionic componentsincorporated into a polymeric matrix to reduce the permeability of theinterference domain to ionic interferents having the same charge as theionic components. In another embodiment, the interference domain mayinclude a catalyst (for example, peroxidase) for catalyzing a reactionthat removes interferents. U.S. Pat. Nos. 6,413,396 and 6,565,509disclose methods and materials for eliminating interfering species.

In certain embodiments, the interference domain may include a thinmembrane that is designed to limit diffusion of certain species, forexample, those greater than 34 kD in molecular weight. In theseembodiments, the interference domain permits certain substances (forexample, hydrogen peroxide) that are to be measured by the electrodes topass through, and prevents passage of other substances, such aspotentially interfering substances. In one embodiment, the interferencedomain is constructed of polyurethane. In an alternative embodiment, theinterference domain comprises a high oxygen soluble polymer, such assilicone.

In some embodiments, the interference domain is formed from one or morecellulosic derivatives. In general, cellulosic derivatives may includepolymers such as cellulose acetate, cellulose acetate butyrate,2-hydroxyethyl cellulose, cellulose acetate phthalate, cellulose acetatepropionate, cellulose acetate trimellitate, or blends and combinationsthereof.

In some alternative embodiments, other polymer types that can beutilized as a base material for the interference domain includepolyurethanes, polymers having pendant ionic groups, and polymers havingcontrolled pore size, for example. In one such alternative embodiment,the interference domain includes a thin, hydrophobic membrane that isnon-swellable and restricts diffusion of low molecular weight species.The interference domain is permeable to relatively low molecular weightsubstances, such as hydrogen peroxide, but restricts the passage ofhigher molecular weight substances, including glucose and ascorbic acid.Other systems and methods for reducing or eliminating interferencespecies that can be applied to the membrane system of the preferredembodiments are described in U.S. Pat. No. 7,074,307, U.S. PatentPublication No. US-2005-0176136-A1, U.S. Pat. No. 7,081,195, and U.S.Patent Publication No. US-2005-0143635-A1, each of which is incorporatedby reference herein in its entirety.

It is contemplated that in some embodiments, the thickness of theinterference domain may be from about 0.01 microns or less to about 20microns or more. In some of these embodiments, the thickness of theinterference domain may be between about 0.01, 0.05, 0.1, 0.15, 0.2,0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns andabout 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5microns. In some of these embodiments, the thickness of the interferencedomain may be from about 0.2, 0.4, 0.5, or 0.6, microns to about 0.8,0.9, 1, 1.5, 2, 3, or 4 microns.

In general, the membrane systems described herein may be formed ordeposited on the exposed electroactive surfaces (e.g., one or more ofthe working and reference electrodes) using known thin film techniques(for example, casting, spray coating, drawing down, electro-depositing,dip coating, and the like), however casting or other known applicationtechniques can also be utilized. In some embodiments, the interferencedomain may be deposited by spray or dip coating. In one exemplaryembodiment, the interference domain is formed by dip coating the sensorinto an interference domain solution using an insertion rate of fromabout 0.5 inch/min to about 60 inches/min, and sometimes about 1inch/min; a dwell time of from about 0.01 minutes to about 2 minutes,and sometimes about 1 minute; and a withdrawal rate of from about 0.5inch/minute to about 60 inches/minute, and sometimes about 1inch/minute; and curing (drying) the domain from about 1 minute to about14 hours, and sometimes from about 3 minutes to about 15 minutes (andcan be accomplished at room temperature or under vacuum (e.g., 20 to 30mmHg)). In one exemplary embodiment including a cellulose acetatebutyrate interference domain, a 3-minute cure (i.e., dry) time is usedbetween each layer applied. In another exemplary embodiment employing acellulose acetate interference domain, a 15 minute cure time is usedbetween each layer applied.

In some embodiments, the dip process can be repeated at least one timeand up to 10 times or more. In other embodiments, only one dip ispreferred. The preferred number of repeated dip processes may dependupon the cellulosic derivative(s) used, their concentration, conditionsduring deposition (e.g., dipping) and the desired thickness (e.g.,sufficient thickness to provide functional blocking of certaininterferents), and the like. In one embodiment, an interference domainis formed from three layers of cellulose acetate butyrate. In anotherembodiment, an interference domain is formed from 10 layers of celluloseacetate. In yet another embodiment, an interference domain is formedfrom 1 layer of a blend of cellulose acetate and cellulose acetatebutyrate. In alternative embodiments, the interference domain can beformed using any known method and combination of cellulose acetate andcellulose acetate butyrate, as will be appreciated by one skilled in theart.

Electrode Domain

It is contemplated that in some embodiments, such as the embodimentillustrated in FIG. 2C, an optional electrode domain 36, also referredto as the electrode layer, may be provided, in addition to thebioprotective domain and the enzyme domain; however, in otherembodiments, the functionality of the electrode domain may beincorporated into the bioprotective domain so as to provide a unitarydomain that includes the functionality of the bioprotective domain,diffusion resistance domain, enzyme domain, and electrode domain.

In some embodiments, the electrode domain is located most proximal tothe electrochemically reactive surfaces. To facilitate electrochemicalreaction, the electrode domain may include a semipermeable coating thatmaintains hydrophilicity at the electrochemically reactive surfaces ofthe sensor interface. The electrode domain can enhance the stability ofan adjacent domain by protecting and supporting the material that makesup the adjacent domain. The electrode domain may also facilitate instabilizing the operation of the device by overcoming electrode start-upproblems and drifting problems caused by inadequate electrolyte. Thebuffered electrolyte solution contained in the electrode domain may alsoprotect against pH-mediated damage that can result from the formation ofa large pH gradient between the substantially hydrophobic interferencedomain and the electrodes due to the electrochemical activity of theelectrodes.

In some embodiments, the electrode domain includes a flexible,water-swellable, substantially solid gel-like film (e.g. a hydrogel)having a ‘dry film’ thickness of from about 0.05 microns to about 100microns, and sometimes from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35,0.4, 0.45, 0.5, 1 microns to about 1.5, 2, 2.5, 3, or 3.5, 4, 4.5, 5, 6,6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 13, 14, 15, 16, 17,18, 19, 19.5, 20, 30, 40, 50, 60, 70, 80, 90, or 100 microns. In someembodiments, the thickness of the electrode domain may be from about 2,2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns in the case of atranscutaneously implanted sensor, or from about 6, 7, or 8 microns toabout 9, 10, 11, or 12 microns in the case of a wholly implanted sensor.The term ‘dry film thickness’ as used herein is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the thickness of a cured filmcast from a coating formulation onto the surface of the membrane bystandard coating techniques. The coating formulation may comprise apremix of film-forming polymers and a crosslinking agent and may becurable upon the application of moderate heat.

In certain embodiments, the electrode domain may be formed of a curablemixture of a urethane polymer and a hydrophilic polymer. In some ofthese embodiments, coatings are formed of a polyurethane polymer havinganionic carboxylate functional groups and non-ionic hydrophilicpolyether segments, which are crosslinked in the presence ofpolyvinylpyrrolidone and cured at a moderate temperature of about 50° C.

Particularly suitable for this purpose are aqueous dispersions offully-reacted colloidal polyurethane polymers having cross-linkablecarboxyl functionality (e.g., BAYBOND®; Mobay Corporation). Thesepolymers are supplied in dispersion grades having apolycarbonate-polyurethane backbone containing carboxylate groupsidentified as XW-121 and XW-123; and a polyester-polyurethane backbonecontaining carboxylate groups, identified as XW-110-2. In someembodiments, BAYBOND® 123, an aqueous anionic dispersion of an aliphatepolycarbonate urethane polymer sold as a 35 weight percent solution inwater and co-solvent N-methyl-2-pyrrolidone, may be used.

In some embodiments, the electrode domain is formed from a hydrophilicpolymer that renders the electrode domain equally or more hydrophilicthan an overlying domain (e.g., interference domain, enzyme domain).Such hydrophilic polymers may include, a polyamide, a polylactone, apolyimide, a polylactam, a functionalized polyamide, a functionalizedpolylactone, a functionalized polyimide, a functionalized polylactam orcombinations thereof, for example.

In some embodiments, the electrode domain is formed primarily from ahydrophilic polymer, and in some of these embodiments, the electrodedomain is formed substantially from PVP. PVP is a hydrophilicwater-soluble polymer and is available commercially in a range ofviscosity grades and average molecular weights ranging from about 18,000to about 500,000, under the PVP K® homopolymer series by BASF Wyandotteand by GAF Corporation. In certain embodiments, a PVP homopolymer havingan average molecular weight of about 360,000 identified as PVP-K90 (BASFWyandotte) may be used to form the electrode domain. Also suitable arehydrophilic, film-forming copolymers of N-vinylpyrrolidone, such as acopolymer of N-vinylpyrrolidone and vinyl acetate, a copolymer ofN-vinylpyrrolidone, ethylmethacrylate and methacrylic acid monomers, andthe like.

In certain embodiments, the electrode domain is formed entirely from ahydrophilic polymer. Useful hydrophilic polymers contemplated include,but are not limited to, poly-N-vinylpyrrolidone,poly-N-vinyl-2-piperidone, poly-N-vinyl-2-caprolactam,poly-N-vinyl-3-methyl-2-caprolactam, poly-N-vinyl-3-methyl-2-piperidone,poly-N-vinyl-4-methyl-2-piperidone, poly-N-vinyl-4-methyl-2-caprolactam,poly-N-vinyl-3-ethyl-2-pyrrolidone,poly-N-vinyl-4,5-dimethyl-2-pyrrolidone, polyvinylimidazole,poly-N,N-dimethyl acrylamide, polyvinyl alcohol, polyacrylic acid,polyethylene oxide, poly-2-ethyl-oxazoline, copolymers thereof andmixtures thereof. A blend of two or more hydrophilic polymers may bepreferred in some embodiments.

It is contemplated that in certain embodiments, the hydrophilic polymerused may not be crosslinked, but in other embodiments, crosslinking maybe used and achieved by any of a variety of methods, for example, byadding a crosslinking agent. In some embodiments, a polyurethane polymermay be crosslinked in the presence of PVP by preparing a premix of thepolymers and adding a cross-linking agent just prior to the productionof the membrane. Suitable cross-linking agents contemplated include, butare not limited to, carbodiimides (e.g.1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride, andUCARLNK® XL-25 (Union Carbide)), epoxides and melamine/formaldehyderesins. Alternatively, it is also contemplated that crosslinking may beachieved by irradiation at a wavelength sufficient to promotecrosslinking between the hydrophilic polymer molecules, which isbelieved to create a more tortuous diffusion path through the domain.

The flexibility and hardness of the coating can be varied as desired byvarying the dry weight solids of the components in the coatingformulation. The term ‘dry weight solids’ as used herein is a broadterm, and is to be given its ordinary and customary meaning to a personof ordinary skill in the art (and is not to be limited to a special orcustomized meaning), and refers without limitation to the dry weightpercent based on the total coating composition after the time thecrosslinker is included. In one embodiment, a coating formulation cancontain about 6 to about 20 dry weight percent, preferably about 8 dryweight percent, PVP; about 3 to about 10 dry weight percent, sometimesabout 5 dry weight percent cross-linking agent; and about 70 to about 91weight percent, sometimes about 87 weight percent of a polyurethanepolymer, such as a polycarbonate-polyurethane polymer, for example. Thereaction product of such a coating formulation is referred to herein asa water-swellable cross-linked matrix of polyurethane and PVP.

In some embodiments, underlying the electrode domain is an electrolytephase that when hydrated is a free-fluid phase including a solutioncontaining at least one compound, typically a soluble chloride salt,which conducts electric current. In one embodiment wherein the membranesystem is used with a glucose sensor such as is described herein, theelectrolyte phase flows over the electrodes and is in contact with theelectrode domain. It is contemplated that certain embodiments may useany suitable electrolyte solution, including standard, commerciallyavailable solutions. Generally, the electrolyte phase can have the sameosmotic pressure or a lower osmotic pressure than the sample beinganalyzed. In preferred embodiments, the electrolyte phase comprisesnormal saline.

Bioactive Agents

It is contemplated that any of a variety of bioactive (therapeutic)agents can be used with the analyte sensor systems described herein,such as the analyte sensor system shown in FIG. 1. In some embodiments,the bioactive agent is an anticoagulant. The term ‘anticoagulant’ asused herein is a broad term, and is to be given its ordinary andcustomary meaning to a person of ordinary skill in the art (and is notto be limited to a special or customized meaning), and refers withoutlimitation to a substance the prevents coagulation (e.g. minimizes,reduces, or stops clotting of blood). In these embodiments, theanticoagulant included in the analyte sensor system may preventcoagulation within or on the sensor. Suitable anticoagulants forincorporation into the sensor system include, but are not limited to,vitamin K antagonists (e.g. Acenocoumarol, Clorindione, Dicumarol(Dicoumarol), Diphenadione, Ethyl biscoumacetate, Phenprocoumon,Phenindione, Tioclomarol, or Warfarin), heparin group anticoagulants(e.g. Platelet aggregation inhibitors: Antithrombin III, Bemiparin,Dalteparin, Danaparoid, Enoxaparin, Heparin, Nadroparin, Parnaparin,Reviparin, Sulodexide, Tinzaparin), other platelet aggregationinhibitors (e.g. Abciximab, Acetylsalicylic acid (Aspirin), Aloxiprin,Beraprost, Ditazole, Carbasalate calcium, Cloricromen, Clopidogrel,Dipyridamole, Epoprostenol, Eptifibatide, Indobufen, Iloprost,Picotamide, Ticlopidine, Tirofiban, Treprostinil, Triflusal), enzymes(e.g., Alteplase, Ancrod, Anistreplase, Brinase, Drotrecogin alfa,Fibrinolysin, Protein C, Reteplase, Saruplase, Streptokinase,Tenecteplase, Urokinase), direct thrombin inhibitors (e.g., Argatroban,Bivalirudin, Desirudin, Lepirudin, Melagatran, Ximelagatran, otherantithrombotics (e.g., Dabigatran, Defibrotide, Dermatan sulfate,Fondaparinux, Rivaroxaban), and the like.

In one embodiment, heparin is incorporated into the analyte sensorsystem, for example by dipping or spraying. While not wishing to bebound by theory, it is believed that heparin coated on the catheter orsensor may prevent aggregation and clotting of blood on the analytesensor system, thereby preventing thromboembolization (e.g., preventionof blood flow by the thrombus or clot) or subsequent complications. Inanother embodiment, an antimicrobial is coated on the catheter (inner orouter diameter) or sensor.

In some embodiments, an antimicrobial agent may be incorporated into theanalyte sensor system. The antimicrobial agents contemplated mayinclude, but are not limited to, antibiotics, antiseptics, disinfectantsand synthetic moieties, and combinations thereof, and other agents thatare soluble in organic solvents such as alcohols, ketones, ethers,aldehydes, acetonitrile, acetic acid, methylene chloride and chloroform.The amount of each antimicrobial agent used to impregnate the medicaldevice varies to some extent, but is at least of an effectiveconcentration to inhibit the growth of bacterial and fungal organisms,such as staphylococci, gram-positive bacteria, gram-negative bacilli andCandida.

In some embodiments, an antibiotic may be incorporated into the analytesensor system. Classes of antibiotics that can be used includetetracyclines (e.g., minocycline), rifamycins (e.g., rifampin),macrolides (e.g., erythromycin), penicillins (e.g., nafeillin),cephalosporins (e.g., cefazolin), other beta-lactam antibiotics (e.g.,imipenem, aztreonam), aminoglycosides (e.g., gentamicin),chloramphenicol, sulfonamides (e.g., sulfamethoxazole), glycopeptides(e.g., vancomycin), quinolones (e.g., ciprofloxacin), fusidic acid,trimethoprim, metronidazole, clindamycin, mupirocin, polyenes (e.g.,amphotericin B), azoles (e.g., fluconazole), and beta-lactam inhibitors(e.g., sulbactam).

Examples of specific antibiotics that can be used include minocycline,rifampin, erythromycin, nafcillin, cefazolin, imipenem, aztreonam,gentamicin, sulfamethoxazole, vancomycin, ciprofloxacin, trimethoprim,metronidazole, clindamycin, teicoplanin, mupirocin, azithromycin,clarithromycin, ofloxacin, lomefloxacin, norfloxacin, nalidixic acid,sparfloxacin, pefloxacin, amifloxacin, enoxacin, fleroxacin,temafloxacin, tosufloxacin, clinafloxacin, sulbactam, clavulanic acid,amphotericin B, fluconazole, itraconazole, ketoconazole, and nystatin.

In some embodiments, an antiseptic or disinfectant may be incorporatedinto the analyte sensor system. Examples of antiseptics anddisinfectants are hexachlorophene, cationic bisiguanides (e.g.chlorhexidine, cyclohexidine) iodine and iodophores (e.g.povidoneiodine), para-chloro-meta-xylenol, triclosan, furan medicalpreparations (e.g. nitrofurantoin, nitrofurazone), methenamine,aldehydes (glutaraldehyde, formaldehyde) and alcohols. Other examples ofantiseptics and disinfectants will readily suggest themselves to thoseof ordinary skill in the art.

In some embodiments, an anti-barrier cell agent may be incorporated intothe analyte sensor system. Anti-barrier cell agents may includecompounds exhibiting effects on macrophages and foreign body giant cells(FBGCs). It is believed that anti-barrier cell agents prevent closure ofthe barrier to solute transport presented by macrophages and FBGCs atthe device-tissue interface during FBC maturation. Anti-barrier cellagents may provide anti-inflammatory or immunosuppressive mechanismsthat affect the wound healing process, for example, healing of the woundcreated by the incision into which an implantable device is inserted.Cyclosporine, which stimulates very high levels of neovascularizationaround biomaterials, can be incorporated into a bioprotective membraneof a preferred embodiment (see U.S. Pat. No. 5,569,462 to Martinson etal.). Alternatively, Dexamethasone, which abates the intensity of theFBC response at the tissue-device interface, can be incorporated into abioprotective membrane of a preferred embodiment. Alternatively,Rapamycin, which is a potent specific inhibitor of some macrophageinflammatory functions, can be incorporated into a bioprotectivemembrane of a preferred embodiment.

In some embodiments, an, anti-inflammatory agent may be incorporatedinto the analyte sensor system to reduce acute or chronic inflammationadjacent to the implant or to decrease the formation of a FBC capsule toreduce or prevent barrier cell layer formation, for example. Suitableanti-inflammatory agents include but are not limited to, for example,nonsteroidal anti-inflammatory drugs (NSAIDS) such as acetometaphen,aminosalicylic acid, aspirin, celecoxib, choline magnesiumtrisalicylate, diclofenac potassium, diclofenac sodium, diflunisal,etodolac, fenoprofen, flurbiprofen, ibuprofen, indomethacin, interleukin(IL)-10, IL-6 mutein, anti-IL-6 iNOS inhibitors (for example, L-NAME orL-NMDA), Interferon, ketoprofen, ketorolac, leflunomide, melenamic acid,mycophenolic acid, mizoribine, nabumetone, naproxen, naproxen sodium,oxaprozin, piroxicam, rofecoxib, salsalate, sulindac, and tolmetin; andcorticosteroids such as cortisone, hydrocortisone, methylprednisolone,prednisone, prednisolone, betamethesone, beclomethasone dipropionate,budesonide, dexamethasone sodium phosphate, flunisolide, fluticasonepropionate, paclitaxel, tacrolimus, tranilast, triamcinolone acetonide,betamethasone, fluocinolone, fluocinonide, betamethasone dipropionate,betamethasone valerate, desonide, desoximetasone, fluocinolone,triamcinolone, triamcinolone acetonide, clobetasol propionate, anddexamethasone.

In some embodiments, an immunosuppressive or immunomodulatory agent maybe incorporated into the analyte sensor system in order to interferedirectly with several key mechanisms necessary for involvement ofdifferent cellular elements in the inflammatory response. Suitableimmunosuppressive and immunomodulatory agents include, but are notlimited to, anti-proliferative, cell-cycle inhibitors, (for example,paclitaxel, cytochalasin D, infiximab), taxol, actinomycin, mitomycin,thospromote VEGF, estradiols, NO donors, QP-2, tacrolimus, tranilast,actinomycin, everolimus, methothrexate, mycophenolic acid, angiopeptin,vincristing, mitomycine, statins, C MYC antisense, sirolimus (andanalogs), RestenASE, 2-chloro-deoxyadenosine, PCNA Ribozyme, batimstat,prolyl hydroxylase inhibitors, PPARγ ligands (for example troglitazone,rosiglitazone, pioglitazone), halofuginone, C-proteinase inhibitors,probucol, BCP671, EPC antibodies, catchins, glycating agents, endothelininhibitors (for example, Ambrisentan, Tesosentan, Bosentan), Statins(for example, Cerivasttin), E. coli heat-labile enterotoxin, andadvanced coatings.

In some embodiments, an anti-infective agent may be incorporated intothe analyte sensor system. In general, anti-infective agents aresubstances capable of acting against infection by inhibiting the spreadof an infectious agent or by killing the infectious agent outright,which can serve to reduce an immuno-response without an inflammatoryresponse at the implant site, for example. Anti-infective agentsinclude, but are not limited to, anthelmintics (e.g. mebendazole),antibiotics (e.g. aminoclycosides, gentamicin, neomycin, tobramycin),antifungal antibiotics (e.g. amphotericin b, fluconazole, griseofulvin,itraconazole, ketoconazole, nystatin, micatin, tolnaftate),cephalosporins (e.g. cefaclor, cefazolin, cefotaxime, ceftazidime,ceftriaxone, cefuroxime, cephalexin), beta-lactam antibiotics (e.g.cefotetan, meropenem), chloramphenicol, macrolides (e.g. azithromycin,clarithromycin, erythromycin), penicillins (e.g. penicillin G sodiumsalt, amoxicillin, ampicillin, dicloxacillin, nafcillin, piperacillin,ticarcillin), tetracyclines (e.g. doxycycline, minocycline,tetracycline), bacitracin, clindamycin, colistimethate sodium, polymyxinb sulfate, vancomycin, antivirals (e.g. acyclovir, amantadine,didanosine, efavirenz, foscarnet, ganciclovir, indinavir, lamivudine,nelfinavir, ritonavir, saquinavir, silver, stavudine, valacyclovir,valganciclovir, zidovudine), quinolones (e.g. ciprofloxacin,levofloxacin); sulfonamides (e.g. sulfadiazine, sulfisoxazole), sulfones(e.g. dapsone), furazolidone, metronidazole, pentamidine, sulfanilamidumcrystallinum, gatifloxacin, and sulfamethoxazole/trimethoprim.

In some embodiments, a vascularization agent may be incorporated intothe analyte sensor system. Vascularization agents generally may includesubstances with direct or indirect angiogenic properties. In some cases,vascularization agents may additionally affect formation of barriercells in vivo. By indirect angiogenesis, it is meant that theangiogenesis can be mediated through inflammatory or immune stimulatorypathways. It is not fully known how agents that induce localvascularization indirectly inhibit barrier-cell formation; however,while not wishing to be bound by theory, it is believed that somebarrier-cell effects can result indirectly from the effects ofvascularization agents.

Vascularization agents may provide mechanisms that promoteneovascularization and accelerate wound healing around the membrane orminimize periods of ischemia by increasing vascularization close to thetissue-device interface. Sphingosine-1-Phosphate (S1P), a phospholipidpossessing potent angiogenic activity, may be incorporated into thebioprotective membrane. Monobutyrin, a vasodilator and angiogenic lipidproduct of adipocytes, may also be incorporated into the bioprotectivemembrane. In another embodiment, an anti-sense molecule (for example,thrombospondin-2 anti-sense), which may increase vascularization, isincorporated into a bioprotective membrane.

Vascularization agents may provide mechanisms that promote inflammation,which is believed to cause accelerated neovascularization and woundhealing in vivo. In one embodiment, a xenogenic carrier, for example,bovine collagen, which by its foreign nature invokes an immune response,stimulates neovascularization, and is incorporated into a bioprotectivemembrane of some embodiments. In another embodiment, Lipopolysaccharide,an immunostimulant, may be incorporated into a bioprotective membrane.In another embodiment, a protein, for example, a bone morphogeneticprotein (BMP), which is known to modulate bone healing in tissue, may beincorporated into the bioprotective membrane.

In some embodiments, an angiogenic agent may be incorporated into theanalyte sensor system. Angiogenic agents are substances capable ofstimulating neovascularization, which can accelerate and sustain thedevelopment of a vascularized tissue bed at the tissue-device interface,for example. Angiogenic agents include, but are not limited to, BasicFibroblast Growth Factor (bFGF), (also known as Heparin Binding GrowthFactor-II and Fibroblast Growth Factor II), Acidic Fibroblast GrowthFactor (aFGF), (also known as Heparin Binding Growth Factor-I andFibroblast Growth Factor-I), Vascular Endothelial Growth Factor (VEGF),Platelet Derived Endothelial Cell Growth Factor BB (PDEGF-BB),Angiopoietin-1, Transforming Growth Factor Beta (TGF-β), TransformingGrowth Factor Alpha (TGF-Alpha), Hepatocyte Growth Factor, TumorNecrosis Factor-Alpha (TNFα), Placental Growth Factor (PLGF),Angiogenin, Interleukin-8 (IL-8), Hypoxia Inducible Factor-I (HIF-1),Angiotensin-Converting Enzyme (ACE) Inhibitor Quinaprilat, Angiotropin,Thrombospondin, Peptide KGHK, Low Oxygen Tension, Lactic Acid, Insulin,Copper Sulphate, Estradiol, prostaglandins, cox inhibitors, endothelialcell binding agents (for example, decorin or vimentin), glenipin,hydrogen peroxide, nicotine, and Growth Hormone.

In some embodiments, a pro-inflammatory agent may be incorporated intothe analyte sensor system. Pro-inflammatory agents are generallysubstances capable of stimulating an immune response in host tissue,which can accelerate or sustain formation of a mature vascularizedtissue bed. For example, pro-inflammatory agents are generally irritantsor other substances that induce chronic inflammation and chronicgranular response at the wound-site. While not wishing to be bound bytheory, it is believed that formation of high tissue granulation inducesblood vessels, which supply an adequate or rich supply of analytes tothe device-tissue interface. Pro-inflammatory agents include, but arenot limited to, xenogenic carriers, Lipopolysaccharides, S. aureuspeptidoglycan, and proteins.

These bioactive agents can be used alone or in combination. Thebioactive agents can be dispersed throughout the material of the sensor,for example, incorporated into at least a portion of the membranesystem, or incorporated into the device (e.g., housing) and adapted todiffuse through the membrane.

There are a variety of systems and methods by which a bioactive agentmay be incorporated into the sensor membrane. In some embodiments, thebioactive agent may be incorporated at the time of manufacture of themembrane system. For example, the bioactive agent can be blended priorto curing the membrane system, or subsequent to membrane systemmanufacture, for example, by coating, imbibing, solvent-casting, orsorption of the bioactive agent into the membrane system. Although insome embodiments the bioactive agent is incorporated into the membranesystem, in other embodiments the bioactive agent can be administeredconcurrently with, prior to, or after insertion of the device in vivo,for example, by oral administration, or locally, by subcutaneousinjection near the implantation site. A combination of bioactive agentincorporated in the membrane system and bioactive agent administrationlocally or systemically can be preferred in certain embodiments.

In general, a bioactive agent can be incorporated into the membranesystem, or incorporated into the device and adapted to diffusetherefrom, in order to modify the in vivo response of the host to themembrane. In some embodiments, the bioactive agent may be incorporatedonly into a portion of the membrane system adjacent to the sensingregion of the device, over the entire surface of the device except overthe sensing region, or any combination thereof, which can be helpful incontrolling different mechanisms or stages of in vivo response (e.g.,thrombus formation). In some alternative embodiments however, thebioactive agent may be incorporated into the device proximal to themembrane system, such that the bioactive agent diffuses through themembrane system to the host circulatory system.

The bioactive agent can include a carrier matrix, wherein the matrixincludes one or more of collagen, a particulate matrix, a resorbable ornon-resorbable matrix, a controlled-release matrix, or a gel. In someembodiments, the carrier matrix includes a reservoir, wherein abioactive agent is encapsulated within a microcapsule. The carriermatrix can include a system in which a bioactive agent is physicallyentrapped within a polymer network. In some embodiments, the bioactiveagent is cross-linked with the membrane system, while in others thebioactive agent is sorbed into the membrane system, for example, byadsorption, absorption, or imbibing. The bioactive agent can bedeposited in or on the membrane system, for example, by coating,filling, or solvent casting. In certain embodiments, ionic and nonionicsurfactants, detergents, micelles, emulsifiers, demulsifiers,stabilizers, aqueous and oleaginous carriers, solvents, preservatives,antioxidants, or buffering agents are used to incorporate the bioactiveagent into the membrane system. The bioactive agent can be incorporatedinto a polymer using techniques such as described above, and the polymercan be used to form the membrane system, coatings on the membranesystem, portions of the membrane system, or any portion of the sensorsystem.

The membrane system can be manufactured using techniques known in theart. The bioactive agent can be sorbed into the membrane system, forexample, by soaking the membrane system for a length of time (forexample, from about an hour or less to about a week, or more preferablyfrom about 4, 8, 12, 16, or 20 hours to about 1, 2, 3, 4, 5, or 7 days).

The bioactive agent can be blended into uncured polymer prior to formingthe membrane system. The membrane system is then cured and the bioactiveagent thereby cross-linked or encapsulated within the polymer that formsthe membrane system.

In yet another embodiment, microspheres are used to encapsulate thebioactive agent. The microspheres can be formed of biodegradablepolymers, most preferably synthetic polymers or natural polymers such asproteins and polysaccharides. As used herein, the term polymer is usedto refer to both to synthetic polymers and proteins. U.S. Pat. No.6,281,015, discloses some systems and methods that can be used inconjunction with the preferred embodiments. In general, bioactive agentscan be incorporated in (1) the polymer matrix forming the microspheres,(2) microparticle(s) surrounded by the polymer which forms themicrospheres, (3) a polymer core within a protein microsphere, (4) apolymer coating around a polymer microsphere, (5) mixed in withmicrospheres aggregated into a larger form, or (6) a combinationthereof. Bioactive agents can be incorporated as particulates or byco-dissolving the factors with the polymer. Stabilizers can beincorporated by addition of the stabilizers to the factor solution priorto formation of the microspheres.

The bioactive agent can be incorporated into a hydrogel and coated orotherwise deposited in or on the membrane system. Some hydrogelssuitable for use in the preferred embodiments include cross-linked,hydrophilic, three-dimensional polymer networks that are highlypermeable to the bioactive agent and are triggered to release thebioactive agent based on a stimulus.

The bioactive agent can be incorporated into the membrane system bysolvent casting, wherein a solution including dissolved bioactive agentis disposed on the surface of the membrane system, after which thesolvent is removed to form a coating on the membrane surface.

The bioactive agent can be compounded into a plug of material, which isplaced within the device, such as is described in U.S. Pat. Nos.4,506,680 and 5,282,844. In some embodiments, it is preferred to disposethe plug beneath a membrane system; in this way, the bioactive agent iscontrolled by diffusion through the membrane, which provides a mechanismfor sustained-release of the bioactive agent in the host.

Release of Bioactive Agents

Numerous variables can affect the pharmacokinetics of bioactive agentrelease. The bioactive agents of the preferred embodiments can beoptimized for short- or long-term release. In some embodiments, thebioactive agents of the preferred embodiments are designed to aid orovercome factors associated with short-term effects (e.g. acuteinflammation or thrombosis) of sensor insertion. In some embodiments,the bioactive agents of the preferred embodiments are designed to aid orovercome factors associated with long-term effects, for example, chronicinflammation or build-up of fibrotic tissue or plaque material. In someembodiments, the bioactive agents of the preferred embodiments combineshort- and long-term release to exploit the benefits of both.

As used herein, ‘controlled,’ ‘sustained’ or ‘extended’ release of thefactors can be continuous or discontinuous, linear or non-linear. Thiscan be accomplished using one or more types of polymer compositions,drug loadings, selections of excipients or degradation enhancers, orother modifications, administered alone, in combination or sequentiallyto produce the desired effect.

Short-term release of the bioactive agent in the preferred embodimentsgenerally refers to release over a period of from about a few minutes orhours to about 2, 3, 4, 5, 6, or 7 days or more.

Loading of Bioactive Agents

The amount of loading of the bioactive agent into the membrane systemcan depend upon several factors. For example, the bioactive agent dosageand duration can vary with the intended use of the membrane system, forexample, the intended length of use of the device and the like;differences among patients in the effective dose of bioactive agent;location and methods of loading the bioactive agent; and release ratesassociated with bioactive agents and optionally their carrier matrix.Therefore, one skilled in the art will appreciate the variability in thelevels of loading the bioactive agent, for the reasons described above.

In some embodiments, in which the bioactive agent is incorporated intothe membrane system without a carrier matrix, the preferred level ofloading of the bioactive agent into the membrane system can varydepending upon the nature of the bioactive agent. The level of loadingof the bioactive agent is preferably sufficiently high such that abiological effect (e.g., thrombosis prevention) is observed. Above thisthreshold, the bioactive agent can be loaded into the membrane system soas to imbibe up to 100% of the solid portions, cover all accessiblesurfaces of the membrane, or fill up to 100% of the accessible cavityspace. Typically, the level of loading (based on the weight of bioactiveagent(s), membrane system, and other substances present) is from about 1ppm or less to about 1000 ppm or more, preferably from about 2, 3, 4, or5 ppm up to about 10, 25, 50, 75, 100, 200, 300, 400, 500, 600, 700,800, or 900 ppm. In certain embodiments, the level of loading can be 1wt. % or less up to about 50 wt. % or more, preferably from about 2, 3,4, 5, 6, 7, 8, 9, 10, 15, or 20 wt. % up to about 25, 30, 35, 40, or 45wt. %.

When the bioactive agent is incorporated into the membrane system with acarrier matrix, such as a gel, the gel concentration can be optimized,for example, loaded with one or more test loadings of the bioactiveagent. It is generally preferred that the gel contain from about 0.1 orless to about 50 wt. % or more of the bioactive agent(s), preferablyfrom about 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 wt. % to about 6,7, 8, 9, 10, 15, 20, 25, 30, 35, 40, or 45 wt. % or more bioactiveagent(s), more preferably from about 1, 2, or 3 wt. % to about 4 or 5wt. % of the bioactive agent(s). Substances that are not bioactive canalso be incorporated into the matrix.

Referring now to microencapsulated bioactive agents, the release of theagents from these polymeric systems generally occurs by two differentmechanisms. The bioactive agent can be released by diffusion throughaqueous filled channels generated in the dosage form by the dissolutionof the agent or by voids created by the removal of the polymer solventor a pore forming agent during the original micro-encapsulation.Alternatively, release can be enhanced due to the degradation of theencapsulating polymer. With time, the polymer erodes and generatesincreased porosity and microstructure within the device. This createsadditional pathways for release of the bioactive agent.

In some embodiments, the sensor is designed to be bioinert, e.g. by theuse of bioinert materials. Bioinert materials do not substantially causeany response from the host. As a result, cells can live adjacent to thematerial but do not form a bond with it. Bioinert materials include butare not limited to alumina, zirconia, titanium oxide or other bioinertmaterials generally used in the ‘catheter/catheterization’ art. Whilenot wishing to be bound by theory, it is believed that inclusion of abioinert material in or on the sensor can reduce attachment of bloodcells or proteins to the sensor, thrombosis or other host reactions tothe sensor.

EXAMPLES Example 1

Transcutaneous sensors, with electrode, enzyme, andbioprotective/diffusion resistance domains, were built and tested. Testsensors were built as described in the section entitled ‘ExemplaryGlucose Sensor Configuration,’ and included an enzyme domain comprisinga Hauthane polyurethane from Hauthaway), PVP (about 18% by weight), andglucose oxidase. Control sensors with no PVP in the enzyme domain werealso built using Hauthane polyurethane in the enzyme domain.

The sensors comprising PVP in the enzyme domain were confirmed to beoperable and capable of measuring glucose with a sensitivity of betweenabout 30 and 80 pA/mg/dL.

Example 2

Sensors built as described in Example 1 were further evaluated todetermine the effect of PVP in the enzyme domain on the difference insensor performance in low versus high oxygen conditions. The differencein signal, which is indicative of sensor drift, was determined bymeasuring the signals produced by control and test sensors under ambientoxygen conditions and under low oxygen conditions (i.e., with oxygenconcentrations of 0.25 mg/L). Differences in signal strength relative toglucose sensitivities for the three sensors are seen in FIG. 4.

Control sensors (i.e., sensors without PVP in the enzyme domain)operated with a glucose sensitivity at between about 50-65 pA/mg/dL, anddemonstrated about 6 to 11% signal loss when operating in low oxygenconditions relative to their performance under ambient conditions.Conversely, test sensors comprising PVP in the enzyme layersdemonstrated increased signal strength when operating in low oxygenconditions relative to their performance under ambient conditions. Infact, the test sensors exhibited up to about 5% signal increase at aneven higher glucose sensitivity (about 60-85 pA/mg/dL).

Accordingly, glucose sensors with enzyme domains comprising polyurethaneand PVP demonstrated improved performance under low oxygen conditionscompared to sensors without PVP in the enzyme domain.

Example 3

Additional testing was conducted to further evaluate relative sensorperformance under high and low oxygen conditions as a function of theamount of PVP in the enzyme domain.

A series of sensors with varying amounts of PVP in the enzyme layer (0,5, 10, 15, 20, and 25 dry wt. % PVP) were prepared. Measurements weregenerated with each of these sensors for samples containing about 400mg/L glucose. The sensors all operated with a sensitivity of about 60pA/mg/dL, with measurements taken under ambient oxygen conditions (i.e.,about 5-10 mg/L oxygen) and under low oxygen conditions (i.e., about0.25 mg/L). As illustrated in FIG. 5, all the test sensors with PVP inthe enzyme domain showed reduced change in current response/signal. Infact, even the test sensor with 5% PVP showed substantial improvementover the control sensor (with no PVP) with respect to loss of signalattributed to being present in a low oxygen environment.

As illustrated in FIG. 6, the control sensor (with 0% PVP in the enzymelayer) exhibited about a 56% decrease in signal in the low oxygenconditions relative to the ambient oxygen conditions. By comparison, allsensors with PVP-containing enzyme layers exhibited greatly improved lowoxygen performance.

Example 4

Sensors are built as described in the section entitled ‘ExemplaryGlucose Sensor Configuration,’ and included an enzyme domain comprisinga polyurethane, glucose oxidase, and glycine betaine at an amountsufficient to provide some degree of enzyme stabilization againstthermal, pH, or other chemical degradation.

As enzymes are denatured at elevated temperatures, the manufacture ofpolymer-based enzymatic glucose sensors requires curing the polymermembrane system at temperatures at or below which enzyme activity ispreserved. The inclusion of glycine betaine in the enzyme domainenhances the thermal stability of glucose oxidase to allow for curingthe polymer membrane system at temperatures that are 5° C., 10° C., 15°C., or even 20° C. or higher than temperatures possible in the absenceof glycine betaine. Because of the increased curing temperature, sensorswith glycine betaine in the enzyme domain require less curing time,resulting in a faster production rate and decreased cost.

Further, glycine betaine in the enzyme domain serves to extend the shelflife of enzymatic glucose sensors. For example, sensors comprisingglycine betaine in the enzyme domain have a shelf life that is 2 weeks,1 month, 6 months, or longer than the shelf life of sensors withoutglycine betaine in the enzyme domain.

Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. Pat. Nos.4,757,022; 4,994,167; 6,001,067; 6,558,321; 6,702,857; 6,741,877;6,862,465; 6,931,327; 7,074,307; 7,081,195; 7,108,778; 7,110,803;7,134,999; 7,136,689; 7,192,450; 7,226,978; 7,276,029; 7,310,544;7,364,592; 7,366,556; 7,379,765; 7,424,318; 7,460,898; 7,467,003;7,471,972; 7,494,465; 7,497,827; 7,519,408; 7,583,990; 7,591,801;7,599,726; 7,613,491; 7,615,007; 7,632,228; 7,637,868; 7,640,048;7,651,596; 7,654,956; 7,657,297; 7,711,402; 7,713,574; 7,715,893;7,761,130; 7,771,352; 7,774,145; 7,775,975; 7,778,680; 7,783,333;7,792,562; 7,797,028; 7,826,981; 7,828,728; 7,831,287; 7,835,777;7,857,760; 7,860,545; 7,875,293; 7,881,763; 7,885,697; 7,896,809;7,899,511; 7,901,354; 7,905,833; 7,914,450; 7,917,186; 7,920,906;7,925,321; 7,927,274; 7,933,639; 7,935,057; 7,946,984; 7,949,381;7,955,261; 7,959,569; 7,970,448; 7,974,672; 7,976,492; 7,979,104;7,986,986; 7,998,071; 8,000,901; 8,005,524; 8,005,525; 8,010,174;8,027,708; 8,050,731; 8,052,601; 8,053,018; 8,060,173; 8,060,174;8,064,977; 8,073,519; 8,073,520; 8,118,877; 8,128,562; 8,133,178;8,150,488; 8,155,723; 8,160,669; 8,160,671; 8,167,801; 8,170,803;8,195,265; 8,206,297; 8,216,139; 8,229,534; 8,229,535; 8,229,536;8,231,531; 8,233,958; 8,233,959; 8,249,684; 8,251,906; 8,255,030;8,255,032; 8,255,033; 8,257,259; 8,260,393; 8,265,725; 8,275,437;8,275,438; 8,277,713; 8,280,475; 8,282,549; 8,282,550; 8,285,354;8,287,453; 8,290,559; 8,290,560; 8,290,561; 8,290,562; 8,292,810;8,298,142; 8,311,749; 8,313,434; 8,321,149; 8,332,008; 8,346,338;8,364,229; 8,369,919; 8,374,667; 8,386,004; and 8,394,021.

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Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. applicationSer. No. 09/447,227 filed on Nov. 22, 1999 and entitled “DEVICE ANDMETHOD FOR DETERMINING ANALYTE LEVELS”; U.S. application Ser. No.12/828,967 filed on Jul. 1, 2010 and entitled “HOUSING FOR ANINTRAVASCULAR SENSOR”; U.S. application Ser. No. 13/461,625 filed on May1, 2012 and entitled “DUAL ELECTRODE SYSTEM FOR A CONTINUOUS ANALYTESENSOR”; U.S. application Ser. No. 13/594,602 filed on Aug. 24, 2012 andentitled “POLYMER MEMBRANES FOR CONTINUOUS ANALYTE SENSORS”; U.S.application Ser. No. 13/594,734 filed on Aug. 24, 2012 and entitled“POLYMER MEMBRANES FOR CONTINUOUS ANALYTE SENSORS”; U.S. applicationSer. No. 13/607,162 filed on Sep. 7, 2012 and entitled “SYSTEM ANDMETHODS FOR PROCESSING ANALYTE SENSOR DATA FOR SENSOR CALIBRATION”; U.S.application Ser. No. 13/624,727 filed on Sep. 21, 2012 and entitled“SYSTEMS AND METHODS FOR PROCESSING AND TRANSMITTING SENSOR DATA”; U.S.application Ser. No. 13/624,808 filed on Sep. 21, 2012 and entitled“SYSTEMS AND METHODS FOR PROCESSING AND TRANSMITTING SENSOR DATA”; U.S.application Ser. No. 13/624,812 filed on Sep. 21, 2012 and entitled“SYSTEMS AND METHODS FOR PROCESSING AND TRANSMITTING SENSOR DATA”; U.S.application Ser. No. 13/732,848 filed on Jan. 2, 2013 and entitled“ANALYTE SENSORS HAVING A SIGNAL-TO-NOISE RATIO SUBSTANTIALLY UNAFFECTEDBY NON-CONSTANT NOISE”; U.S. application Ser. No. 13/733,742 filed onJan. 3, 2013 and entitled “END OF LIFE DETECTION FOR ANALYTE SENSORS”;U.S. application Ser. No. 13/733,810 filed on Jan. 3, 2013 and entitled“OUTLIER DETECTION FOR ANALYTE SENSORS”; U.S. application Ser. No.13/742,178 filed on Jan. 15, 2013 and entitled “SYSTEMS AND METHODS FORPROCESSING SENSOR DATA”; U.S. application Ser. No. 13/742,694 filed onJan. 16, 2013 and entitled “SYSTEMS AND METHODS FOR PROVIDING SENSITIVEAND SPECIFIC ALARMS”; U.S. application Ser. No. 13/742,841 filed on Jan.16, 2013 and entitled “SYSTEMS AND METHODS FOR DYNAMICALLY ANDINTELLIGENTLY MONITORING A HOST'S GLYCEMIC CONDITION AFTER AN ALERT ISTRIGGERED”; U.S. application Ser. No. 13/747,746 filed on Jan. 23, 2013and entitled “DEVICES, SYSTEMS, AND METHODS TO COMPENSATE FOR EFFECTS OFTEMPERATURE ON IMPLANTABLE SENSORS”; U.S. application Ser. No.13/779,607 filed on Feb. 27, 2013 and entitled “ZWITTERION SURFACEMODIFICATIONS FOR CONTINUOUS SENSORS”; U.S. application Ser. No.13/780,808 filed on Feb. 28, 2013 and entitled “SENSORS FOR CONTINUOUSANALYTE MONITORING, AND RELATED METHODS”; U.S. application Ser. No.13/784,523 filed on Mar. 4, 2013 and entitled “ANALYTE SENSOR WITHINCREASED REFERENCE CAPACITY”; U.S. application Ser. No. 13/789,371filed on Mar. 7, 2013 and entitled “MULTIPLE ELECTRODE SYSTEM FOR ACONTINUOUS ANALYTE SENSOR, AND RELATED METHODS”; U.S. application Ser.No. 13/789,279 filed on Mar. 7, 2013 and entitled “USE OF SENSORREDUNDANCY TO DETECT SENSOR FAILURES”; U.S. application Ser. No.13/789,339 filed on Mar. 7, 2013 and entitled “DYNAMIC REPORT BUILDING”;U.S. application Ser. No. 13/789,341 filed on Mar. 7, 2013 and entitled“REPORTING MODULES”; U.S. application Ser. No. 13/790,281 filed on Mar.8, 2013 and entitled “SYSTEMS AND METHODS FOR MANAGING GLYCEMICVARIABILITY”; U.S. application Ser. No. 13/796,185 filed on Mar. 12,2013 and entitled “SYSTEMS AND METHODS FOR PROCESSING ANALYTE SENSORDATA”; U.S. application Ser. No. 13/796,642 filed on Mar. 12, 2013 andentitled “SYSTEMS AND METHODS FOR PROCESSING ANALYTE SENSOR DATA”; U.S.application Ser. No. 13/801,445 filed on Mar. 13, 2013 and entitled“SYSTEMS AND METHODS FOR LEVERAGING SMARTPHONE FEATURES IN CONTINUOUSGLUCOSE MONITORING”; U.S. application Ser. No. 13/802,424 filed on Mar.13, 2013 and entitled “SYSTEMS AND METHODS FOR LEVERAGING SMARTPHONEFEATURES IN CONTINUOUS GLUCOSE MONITORING”; U.S. application Ser. No.13/802,237 filed on Mar. 13, 2013 and entitled “SYSTEMS AND METHODS FORLEVERAGING SMARTPHONE FEATURES IN CONTINUOUS GLUCOSE MONITORING”; andU.S. application Ser. No. 13/802,317 filed on Mar. 13, 2013 and entitled“SYSTEMS AND METHODS FOR LEVERAGING SMARTPHONE FEATURES IN CONTINUOUSGLUCOSE MONITORING”.

All references cited herein, including but not limited to published andunpublished applications, patents, and literature references, areincorporated herein by reference in their entirety and are hereby made apart of this specification. To the extent publications and patents orpatent applications incorporated by reference contradict the disclosurecontained in the specification, the specification is intended tosupersede and/or take precedence over any such contradictory material.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing, the term ‘including’ shouldbe read to mean ‘including, without limitation’ or the like; the term‘comprising’ as used herein is synonymous with ‘including,’‘containing,’ or ‘characterized by,’ and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps; theterm ‘example’ is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; and adjectivessuch as ‘known,’ ‘normal,’ ‘standard,’ and terms of similar meaningshould not be construed as limiting the item described to a given timeperiod or to an item available as of a given time, but instead should beread to encompass known, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, a groupof items linked with the conjunction ‘and’ should not be read asrequiring that each and every one of those items be present in thegrouping, but rather should be read as ‘and/or’ unless expressly statedotherwise. Similarly, a group of items linked with the conjunction ‘or’should not be read as requiring mutual exclusivity among that group, butrather should be read as ‘and/or’ unless expressly stated otherwise. Inaddition, as used in this application, the articles ‘a’ and ‘an’ shouldbe construed as referring to one or more than one (i.e., to at leastone) of the grammatical objects of the article. By way of example, ‘anelement’ means one element or more than one element.

The presence in some instances of broadening words and phrases such as‘one or more,’ ‘at least,’ ‘but not limited to,’ or other like phrasesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification are to be understood as beingmodified in all instances by the term ‘about.’ Accordingly, unlessindicated to the contrary, the numerical parameters set forth herein areapproximations that may vary depending upon the desired propertiessought to be obtained. At the very least, and not as an attempt to limitthe application of the doctrine of equivalents to the scope of anyclaims in any application claiming priority to the present application,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

Furthermore, although the foregoing has been described in some detail byway of illustrations and examples for purposes of clarity andunderstanding, it is apparent to those skilled in the art that certainchanges and modifications may be practiced. Therefore, the descriptionand examples should not be construed as limiting the scope of theinvention to the specific embodiments and examples described herein, butrather to also cover all modification and alternatives coming with thetrue scope and spirit of the invention.

What is claimed is:
 1. A device for measurement of an analyteconcentration, the device comprising: a sensor configured to generate asignal indicative of a concentration of an analyte; and a sensingmembrane located over the sensor, the sensing membrane comprising anenzyme domain comprising an enzyme, a base polymer, and a zwitterionicenzyme stabilizing agent, wherein the zwitterionic enzyme stabilizingagent is a betaine.
 2. The device of claim 1, wherein the enzyme domainfurther comprises a hydrophilic polymer, wherein the hydrophilic polymercomprises from about 10 wt. % to about 25 wt. % of the enzyme domain. 3.The device of claim 2, wherein the hydrophilic polymer comprises fromabout 15 wt. % to about 20 wt. % of the enzyme domain.
 4. The device ofclaim 2, wherein the hydrophilic polymer is selected from the groupconsisting of poly-N-vinylpyrrolidone (PVP), poly(ethylene glycol)(PEG), polyacrylamide, acetates, polyethylene oxide (PEO),polyethylacrylate (PEA), poly-N-vinyl-3-ethyl-2-pyrrolidone,poly-N-vinyl-4,5-dimethyl-2-pyrrolidone, poly-N,N-dimethylacrylamide,polyvinyl alcohol, polyvinyl acetate, polymers with pendent ionizablegroups and copolymers or blends thereof.
 5. The device of claim 2,wherein the hydrophilic polymer comprises poly-N-vinylpyrrolidone (PVP).6. The device of claim 1, wherein the enzyme is selected from the groupconsisting of glucose oxidase, glucose dehydrogenase, galactose oxidase,cholesterol oxidase, amino acid oxidase, alcohol oxidase, lactateoxidase, and uricase.
 7. The device of claim 1, wherein the enzyme isglucose oxidase.
 8. The device of claim 1, wherein the base polymercomprises at least one polymer selected from the group consisting ofepoxies, polyolefins, polysiloxanes, polyethers, acrylics, polyesters,carbonates, and polyurethanes.
 9. The device of claim 7, where thepolyurethane comprises a polyurethane copolymer.
 10. The device of claim1, wherein the base polymer comprises a polyurethane.
 11. The device ofclaim 1, wherein the enzyme domain further comprises a cross-linkingagent in an amount sufficient to induce cross-linking between polymermolecules.
 12. The device of claim 11, wherein the cross-linking agentcomprises a cross-linking agent selected from the group consisting ofisocyanate, carbodiimide, gluteraldehyde or other aldehydes, epoxy,acrylates, free-radical based agents, ethylene glycol diglycidyl ether(EGDE), poly(ethylene glycol) diglycidyl ether (PEGDE), and dicumylperoxide (DCP).
 13. The device of claim 11, wherein the cross-linkingagent comprises from about 0.1 wt. % to about 15 wt. % of the total dryweight of the enzyme, cross-linking agent, and polymers.
 14. The deviceof claim 1, wherein the thickness of the enzyme domain is from about0.05 micron to about 100 microns.
 15. The device of claim 1, wherein thesensor comprises an electrode.
 16. The device of claim 1, wherein thesensing membrane further comprises a resistance domain configured tocontrol a flux of the analyte therethrough.
 17. The device of claim 1,wherein the sensing membrane further comprises an interference domainlocated more proximal to the sensor than the enzyme domain, wherein theinterference domain comprises at least about 25% silicone by weight. 18.The device of claim 1, wherein the device is configured for continuousmeasurement of an analyte concentration.
 19. The device of claim 1,wherein the analyte is glucose.
 20. The device of claim 18, wherein thedevice is configured for in vivo glucose measurement.